I used to buy most of my ammo from Palmetto before I started shopping around a bit. Go to Ammo Seek and use their filter to get rid of the high shipping companies. I sort for an "8" or higher on shipping, which includes a minimal shipping fee, a "conditionally free" (minimum purchase) or straight free shipping. I also filter my search for brass case only, new only. Outdoor Limited, SGAmmo and Rivertown Munitions all rate highly and are generally near the top. Right now, you can get 9MM Winchester white box for about 26 cents a round on a 1,000 rounds through Outdoor Limited. PMC .556 running about 44 cents, again on a 1,000 round purchase. Both of these with free shipping.
On the Winchester ammo, Winchester has had two rebate offers over the last six months, the one that just ended was 20% off, up to $75 off any one box of ammo. That put the cost of that 1,000 round 9MM box down to about 20 cents a round. When that deal is out, it's only for one box, so you have to watch and make sure you're not getting two 500 round boxes instead of the single 1,000 round box.
RTR,
Tim