NCAA BB 10 sold dbl of C Hoop 2k8, because that was the only college BB game out. If you add the total number of sales for CHoop2k8 and NCAA BB 8 they're about the same as what NCAA BB 10 had.
I would think that if 2k took there NBA game and turned it into a college game they would sell around 400-600k copies.
NBA 2k sells in the 4mil plus range. If 2k could make a college hoops game that transfers well to the nba game and release the college hoops game before they release the NBA game, then I could see a large increase in sales. If draft classes and player creations could transfer to the NBA then I could see people buying the C Hoops game to create a player/career starting in High school and then through college and then taking them into the NBA game , giving their created player a stat/skill point increase for the NBA