Don't just look at the price, compare prices after shipping and handling. You'd be surprised at how big a difference this can be. And the final price with shipping and handling is what you really care about. I know this seems but obvious, but just wanted to point it out.
The lowest price I've ever seen online is $8.99 / box (in quantities of 10 boxes) and this either for non-reloadable or for Russian ammo. I've never seen any NEW 9mm ammo go for less than that. Most of the time you'll pay about $9.99 / box + shipping.
Don't forget to look locally!
Dick's Sporting Goods:
Another place to look at is your local Dick's Sporting Goods. I've seen Dick's run specials on Remington UMC 9mm for $11.97 local, but if you buy 10 boxes (a case), then the price drops to $9.99. Be sure to ask for their "case price." You just have to wait for the specials (assuming you have a Dick's locally). They run specials on other brands, too, so ask for the price on everything. At $9.99 local, you'll be hard-press to find a better deal online once you factor in tax vs shipping.
Wal-Mart:
They generally have decent prices. I just picked up some Federal Champion 115g FMJ for $10.97 + tax. This is the normal price. I needed some quickly, so I can't complain about that price for retail.
A few more places to try on-line:
Natchez Shooters Supply
Palmetto State Armory
Lastly, look for specials. This President's Day weekend 2012, Cabela's is running a sale on Federal Champion 9mm for $9.99 box and $5 shipping for over $99 with code "22SHIP". There are other calibers on sale too, this weekend only.
Cabelas Federal Champion 9mm