Hands down....Your local Walmart will be cheaper than any mail-order ammo.
Im in houston right now, and Walmart on ST HWY 6, 3 miles south of I-10, has the 100 round Winchester White Box at 16.67.
I just got 1000 rounds for 179 with tax.
Once you add shipping, you will not be able to beat that, unless you are buying steel cased garbage, and I still dont think its that cheap anymore.