There are stocks that are cheap for a good reason and those that are cheap only for a season. Stocks that are cheap for a reason could be value traps -- low-priced and likely to stay that way. Stocks that are cheap for a season, on the other hand, could make patient investors a lot of money over time.

Three big biotech stocks appear to be incredibly cheap right now: AbbVie (NYSE:ABBV), Celgene (NASDAQ:CELG), and Gilead Sciences (NASDAQ:GILD). But why are these stocks inexpensive? 

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AbbVie: Beating back the biosimilars

AbbVie stock trades at less than 10 times forward earnings. That makes the biotech's shares practically a steal considering its prospects for growth. Wall Street analysts project AbbVie will increase its annual earnings by an average of more than 14% over the next five years.

Why is AbbVie trading at such a discount? It's probably due in large part to fears that sales for the company's top-selling drug, Humira, could be in jeopardy. The U.S. Food and Drug Administration (FDA) approved Amgen's biosimilar rival to Humira last year. European biosimilar competition could be on the way as well.

I'm not worried about these threats just yet, though. AbbVie is putting up a fight to keep Amgen at bay. Although Humira's composition patent expired in December 2016, AbbVie has more than 70 other patients for the drug. The company thinks it can protect Humira from competition until 2022.

In the meantime, AbbVie is cranking out products to grow earnings. Sales for Imbruvica are soaring. The cancer drug could generate annual revenue of $5 billion by 2020. AbbVie should also have several more blockbusters from its pipeline over the next few years.  

Celgene: Growth on multiple fronts

Celgene stock looks more expensive than AbbVie does. The biotech's shares are trading at under 14 times forward earnings. But Celgene's growth potential is outstanding. Both the company and Wall Street analysts project average annual growth rates of 22% over the next few years. This growth outlook makes Celgene's stock a bargain, in my view.

It could be that Celgene is cheap because some observers don't fully believe the biotech can grow earnings as much as predicted. I wouldn't bet against Celgene, though. Its top drug, Revlimid, continues to increase sales at a solid pace. Johnson & Johnson's Darzalex could present a challenge to Revlimid down the road, but that's not likely to happen until 2020 or later.

Otezla is providing much of Celgene's growth. The drug already made over $1 billion last year as a treatment for psoriasis. There could be more approved autoimmune disease indications for Otezla on the way.

Celgene's pipeline includes several other strong autoimmune disease candidates. GED-0301 is in a late-stage study for treatment of Crohn's disease, while ozanimod is in a late-stage study for treating ulcerative colitis. Celgene also recently reported positive results from a late-stage study evaluating ozanimod in treating multiple sclerosis.

Gilead Sciences: A hole burning in the biotech's pocket

Gilead Sciences' forward earnings multiple of less than 9 is the lowest of all three biotechs. However, Gilead can't boast about strong growth prospects right now. Actually, with its hepatitis C virus (HCV) franchise struggling, the company seems likely to continue to experience further earnings declines in the next few years.

That outlook could change, though. Gilead reported $32.4 billion of cash, cash equivalents, and marketable securities at the end of 2016. The big biotech also generated cash flow of $16.7 billion last year. Even though that level will undoubtedly drop in 2017, Gilead still has a lot of money that it can spend. And, based on comments by the company's executives, Gilead is on a mission to spend at least some of its cash stockpile.

Gilead CEO John Milligan said at the RBC Capital Markets Healthcare Conference that his company is looking for a catalyst this year. He hinted that the biotech would like to bolster its pipeline in three key areas: oncology, inflammatory diseases, and non-alcoholic steatohepatitis (NASH). 

If Gilead does make some smart acquisitions that improve its earnings growth prospects, the stock probably won't remain as cheap as it is right now. It's a question of when -- not if -- that the company begins using its money to buy growth.

Best bargain

I own all three of these biotech stocks. I like AbbVie for its growth and tremendous dividend yield of more than 4%. I like Celgene for its great growth prospects. I like Gilead Sciences because of its excellent cash flow and cash position (and how the company will use both to reward shareholders).

Which is the best bargain among the three stocks? My vote goes to Celgene. I don't see any huge obstacles in the way for the biotech to achieve its growth goals. And if earnings grow as the company expects they will, Celgene should be a big winner for investors over the next few years.

Keith Speights owns shares of AbbVie, Celgene, and Gilead Sciences. The Motley Fool owns shares of and recommends Celgene and Gilead Sciences. The Motley Fool recommends Johnson and Johnson. The Motley Fool has a disclosure policy.