Up-top under the changing Banner ad, you'll see Home :: Discussion Forums :: Equipment Exchange.
Under Equipment Exchange you'll find a forum for Magazines, suggest you check there and support our local sellers.
Price will vary, but for 30 round USGI mags from $15-24 dollars each and maybe more for those w/ green followers. 20 round USGIs run from $18 all the way up to $40-50 for those w/ Colt floorplates. Shop around and suggest you go to the Magazine discussion forum and read the MAG-Fac tacked to the top of the forum as it contains alot of excellent information about mags.
You're late in the game and mag prices are way higher now than say 20 years ago, but welcome to the site. BTW, GPSS (Georgia Precision Shooting Supplies) under the Industry forums and other site supporters do sell mags when they have them in stock, so ya might check them out too, (prices maybe little higher but they are TOP quality mags).
Mike