Everyone likes getting something for nothing. When you're trying to pick the best online broker you can find, you'll inevitably field plenty of lucrative offers from companies that want your business and are willing to pay for it.

Yet before you grab the opportunity to score valuable prizes, it's important to make sure you weigh the costs you'll bear from an ongoing relationship with a broker against the value of whatever freebies you get. Often, one-time gifts won't be worth the price you'll eventually pay.

Paying you to come in the door
Looking at the offers that many online brokers make, you can break them down into a couple of categories. Some companies offer up-front cash for you to sign up and fund your account, while others will give you discounts on commissions and other costs for a certain period after signing up.

In the cash category, some of the amounts you'll see thrown around are pretty staggering. For instance, just a look at websites reveals some interesting offers. E*TRADE Financial (ETFC) currently offers on its homepage as much as $600 to open a retirement account funded with $250,000 or more, with smaller offers of $300 for accounts between $100,000 and $250,000 and $100 for accounts between $25,000 and $100,000. TD AMERITRADE (AMTD) matches those cash amounts for any account, without restricting it to retirement products, while Schwab (SCHW 1.31%) has a similar promotion for retirement accounts with a slightly different lower tier, offering $200 for accounts of $50,000 or more. Fidelity takes it a step higher, offering Schwab's tiers but adding on a $1,200 payment for depositing $500,000 or more and a whopping $2,500 for $1 million accounts.

Among other discounts, trading stocks commission-free for a certain period seems to be the most popular perk. Even for accounts that aren't valuable enough to meet its cash-back standards, TD AMERITRADE offers 60 days of commission-free trading for accounts as small as $2,000. Schwab currently offers as much as six months of commission-free equity trading with a $50,000 deposit. Bank of America's (BAC 1.70%) Merrill Edge offers 30 commission-free stock or ETF trades every month if you keep a $25,000 account balance in a self-directed investing account.

After the honeymoon's over
The problem with free offers is that eventually, they go away. To make sure you really have the best online broker, you have to make sure you'll be happy with it even after you take home that extra cash or your free commissions go away.

One way that brokers have distinguished themselves is by offering permanent commission-free trading on exchange-traded funds. For instance, Fidelity offers 30 different iShares ETFs from BlackRock (BLK 0.86%) at no commission, while Schwab and Vanguard both let customers trade their own proprietary ETFs at no charge. TD AMERITRADE has a list of more than 100 different ETFs from multiple providers that it allows its customers to trade at no commission.

Unlike most freebies, policies allowing you to trade ETFs commission-free are valuable not just at the beginning of a brokerage relationship but throughout its course. It may be useful to have commission-free trading as you initially invest a big lump sum, but if you're like most people and add relatively modest amounts to your savings with every paycheck, commissions can eat you alive. Having the ability to put money to work in high-quality, low-cost ETFs can be the foundation of an entire long-term investing strategy, and online brokers are smart to realize that giving up some commission income on the front end can help their customers build a lot more wealth more quickly. At that point, brokerage companies can then take steps to try to get more revenue from their high-end clientele after having built years of loyal service.

Be smart
Before you jump at some free money or other valuable goodies, it's worth taking the time and effort to make sure you're getting the best long-term deal you can find. What happens long after introductory offers have come and gone is what will truly determine whether you've found the best online broker for you.

Looking for a broker? Our broker comparison table is a great place to start, showing a handful of sponsored alternatives in side-by-side fashion.