That depends on what type of ammo you're looking for. There isn't one site that's cheaper on everything. You could look through the links to price the ammo you're looking for.
federal american eagle (not champion) FMJ 124gr, 9mm , 1000 rounds - 1 case 20 boxes of 50 = $220 or $11 per box, I did not have to leave the house, free shipping and $0.00 sales tax, took cc over the phone and asked if they would double the order, they said no problem.
Updated list...This is not an endorsement of any of these companies. Just wanted to give my fellow INGOers some options for online ammo purchases. This is every company that I could find online, but there may be more. If so, please let me know and I will add them to my list.