I am assuming you are a rookie investor as you are comparing apples to oranges. GILD's 5 year estimated growth is 12% per annum. MDVN's is 66%. Using your logic MDVN should be trading at 5 times GILD"s 7x earnings. Of course if you feel these estimates are wrong, which is very possible, you may invest accordingly. Biotechs have been slammed recently for many reasons and hardly at all for whatever promise they have in the future, which may make for a promising opportunity for those withs the guts to invest. Good luck.
MDVN got way ahead of itself price wise when it hit 140. I sold a chunk in the 120's mainly because it became too large of a percentage of my portfolio thanks to a 1000%+ rise in price. I started nibbling back again around 40 (post split) which was in hindsight too soon. As it went to high too fast it is now going down too far and too fast which is typically the case with a smallish biotech. If I didn't own any I would be loading up right now and am indeed adding a few more shares.