Image source: Getty Images.

Buying stocks on sale can be profit-friendly, but not every inexpensive stock is worthy of being in your portfolio. After all, some stocks are cheap for good reason. We asked some of our top contributors to highlight some of their favorite stocks that are on sale. These companies cut across industries, but they all have two things in common. Each has catalysts ahead that can boost the top and bottom line, and their shares can be bought at a reasonable price. Read on to see whether these companies may be right for your portfolio.

Beaten-down bank trading at a big discount

Jason Hall: There are few growth stocks as undervalued as BofI Holding Inc. (AX -0.20%) is, though not without some reason. After all, the company has been subject to numerous allegations over the past year or so, for a variety of things ranging from illegal activity by the CEO to dirty dealings with its underwriting and appraisals, among a litany of other things.

Compounded, they've pushed the internet-only bank's stock down over half from its high and driven its price to book value multiple to levels of banks 70 times bigger:

BOFI Price to Book Value Chart

BOFI Price to Book Value data by YCharts

Bottom line: BofI is a cost-efficient growth machine, and I don't buy the allegations, which, by the way, are all being pushed by anonymous bloggers with short positions (and potentially very large short-selling financial backers). 

Don't get me wrong -- I'm not saying the allegations are false because the accusers are anonymous or short. I'm saying that it's reached a laughable point when the latest short thesis is that someone in the audit department left the company so there must be trouble. I'm just not buying it. 

Either the allegations are right or they're not, and it's very hard to imagine regulatory agencies ignoring a bank if there's this much bad stuff happening. To me that sounds like an excellent opportunity to take a calculated risk on a very undervalued growth dynamo. If you're willing to take on the risk, BofI shares are trading at an incredible discount for a company that's grown earnings like this over the past five years:

BOFI EPS Diluted (TTM) Chart

BOFI EPS Diluted (TTM) data by YCharts

Image source: Biogen Inc.

Recovering its balance

Todd Campbell: Biogen Inc. (BIIB -1.53%) shares took it on the chin earlier this year when management reported that a once-promising multiple sclerosis drug had come up short in trials.

While the news was disappointing, the significant drop in Biogen's shares may have created an opportunity for value-minded investors.

Biogen's various MS drugs capture more than a third of a $19 billion and growing market, and second-quarter sales jumped 12% versus a year ago to $2.9 billion.

Sales growth was supported by ongoing demand for Tecfidera, an oral MS drug that's selling at an annualized $3.9 billion pace, and rising demand for its long-lasting hemophilia drugs, Eloctate and Alprolix. In Q2, Eloctate and Alprolix sales totaled a combined $205 million, up from $128 million in the same quarter last year.

Importantly, Biogen's cost-cutting is helping it transform more of its sales into earnings, and that's keeping its valuation in check. After adjusting for currency and one-time items, non-GAAP EPS grew 23% to $5.21. That performance has Biogen predicting full year non-GAAP EPS will be between $19.70 and $20 in 2016. That means investors can buy shares for less than 14 times this year's earnings.

Overall, because Biogen has one of the best balance sheets in biotech, a multibillion-dollar commitment to share buybacks, and double-digit top- and bottom-line growth, it's one of my favorite undervalued growth stocks.

Image source: Fitbit.

A rebound "fit" for a king

Steve Symington: I think now is a fantastic time for investors to consider buying shares of Fitbit (FIT). After suffering more than a 50% decline so far in 2016, shares of the wearable-fitness tech specialist trade at a reasonable 22 times trailing 12-month earnings, and a mouthwatering 9.1 times next year's expected earnings.

That's not to say the stock fell for no reason; as fellow Fool Demitrios Kalogeropoulos recently pointed out, though Fitbit is still enjoying torrid growth -- revenue last quarter jumped 50% year over year, as the company sold an impressive 4.8 million connected health and fitness devices -- IDC research shows its market share actually fell from 33% to 25% over the past year as new entrants flooded the market with competitive devices.

However, Fitbit has responded with planned increased investments in R&D to accelerate its pace of innovation and widen its competitive moat. And the market seems to be ignoring the fact that last quarter's results were strong enough to allow the company to increase its outlook for the remainder of the year -- albeit not enough to live up to Wall Street's near-term demands. And that's not to mention the fact around 70% of Fitbit's revenue still comes from the U.S., leaving the international market just ripe for the picking. 

All told, though Fitbit's niche won't become less crowded anytime soon, I'm convinced this market leader's recent punishment doesn't fit the crime.

Image source: Getty Images.

The broker is primed for growth

Brian Feroldi: One growth stock that I think is trading for an attractive price today is Interactive Brokers Group (IBKR 1.71%).

As its name suggestions, Interactive Brokers Group is an electronic broker that gives its customers the ability to buy and sell a wide range of financial securities, such as stocks, bonds, forex, and more. However, unlike many other traditional brokers that you're probably more familiar with, Interactive Brokers primarily caters to only highly active traders, such as hedge-fund managers and day-traders. These customers demand speed and low-cost execution since they trade frequently, and on both fronts Interactive Brokers' platform stands head and shoulders above the rest of the crowd. That's a big reason the company has been attracting new clients to its platform at double-digit rates for years.

That's all great, and it makes the company highly profitable, but the brokerage business tends to be lumpy by nature. In the most recent quarter we saw that lumpiness play out, which is one reason the market hasn't been kind to this company's stock recently. Net revenue fell 4.7% year over year because the company's customers aren't trading quite as frequently. In addition, Interactive Brokers' market-making business took a big haircut during the quarter.

Those factors are contributing to the weakness in its share price, but since I'm a believer that both of those trends will reverse themselves over time, I consider this to be a buying opportunity. After all, this past quarter we saw customer accounts and customer equity both grow by double digits. Better yet, the company's scalable business model allowed its adjusted earnings rise to about 8% to $0.40 per share, despite the revenue headwind.

Shares of Interactive Brokers Group are currently down more than 20% from their 52-week, and they can be purchased for about 21 times next year's earnings. I think that's a nice entry point for a company that's expected to grow profits by more than 16% annually over the next five years.

Image source: Getty Images.

A cyclical growth stock

Tim Green: On the surface, NXP Semiconductors (NXPI -1.63%) doesn't look like much of a growth stock at the moment. During the first quarter, NXP reported an 11% year-over-year decline in revenue and a 15% year-over-year decline in non-GAAP EPS after adjusting for the acquisition of Freescale Semiconductor. Growth companies are supposed to grow quickly, and NXP currently fails that test.

But the semiconductor industry is cyclical, with demand rising and falling based on a variety of factors. NXP has no control over these ebbs and flows, and all the company can do is position itself for the eventual recovery. With the acquisition of Freescale, NXP has made itself the largest provider of automotive semiconductor products in the world. With cars getting smarter and more connected, the long-term picture looks bright.

Analysts expect NXP's earnings to be roughly flat this year, with substantial growth returning in fiscal 2017. The average analyst estimate calls for non-GAAP EPS of $7.48 next year, up from an expected $5.75 this year. The analysts could be wrong, of course, as the downturn in demand may last longer than expected. Investors will need to be patient. But with the stock trading for around 11 times expected 2017 earnings, NXP is a growth stock trading at an attractive price.