100$ for a new 300 gig is a very good deal. cheapest harddrives I have seen have been about 50 cents/gig and those were white label drives.
as for game storage, you have to assume that the drive is actually only about 280 gigs give or take. then take 2 gigs for applications (thats a little extreme but still can never be to sure) so 275 gigs we will say left for games. at average of 5 gigs a game that is 55 games.