Magazines are expensive too. Barret 30 rd mags will cost about $40, PRI mags a little less, and C product mags are about $20. The C product mags work very well for me. The PRI mags are a tight fit (I hear they are longer for handloaders) and they don't reliably lock the bolt on the last round for me in a Rock River. The Barrets are great and they are very strong steel, but they are expensive.
Ammo is about $20 per box of 20 while 5.56/.223 is $7 or $8 for 20. .308/7.62 is about $15-20 per 20 for fmj ammo.