Read this article. I doubt 50% of 2/3 star guys get drafted.
It is a few years old, but if I had to guess numbers are not that much different.
50% get drafted with a 75% retention rate. Retention rate = "if a player has made an active roster in two of three or five of seven potential seasons, we'll say he hasn't been retained. If he's made an active roster in three of four or six of eight, we'll say he has."