Wal-Mart. About $9.86/550 rounds.
If you want to try something a little different, but reputed to be more accurate (in some rifles) than many so-called bargain brands, try Aguila from Centurion Distributing in TX. Standard velocity stuff can be had for $122.00 plus freight for a case of five thousand rounds.
John Feamster in PS Magazine a couple of years ago had very favorable things to say about the performance of this ammo vs. the cost.