Here is a 12 pack of gen 3's for $99 with free shipping. Some people dont like these guys but I have ordered XDm mags a few times with great results.
Magpul PMAG Magazines .223 Remington AR15/M4/M16
They have some 210 round battle packs loaded into 7 non window PMAGS for right around 200.Ammunition Depot has pretty good prices on PMAGs. Midway USA is my other go-to online source for them.