Cheaper than dirt has the following:
Product Description: Ammo .223 SS109, 62g 20-Rd Box Military Penetrator FMJ Brass
Catalog Item #: AMM-202 Mfg Item #: AM1106
For $4.07 per box or about $0.20 per round
They also have this:
Product Description: Ammo .223 62gr Hollow Pt 20-Rd Box Nickel Steel Case 20-Rd Bx
Catalog Item #: AMM-228 Mfg Item #: AMM228
For $2.89 a box or about $0.145 per round (This is Russian)
Then we have this in 7.62x39 surplus
Product Description: Ammo 7.62x39 FMJ 700-Rd Tin FMJ 122gr Non-Corr Sealed Tin
Catalog Item #: AMM-637 Mfg Item #: AMM637
For $64.97 or $0.093 per round.
Good luck in your choice. The only thing that is not mentioned here is the difference in down range energy. The .223 at 200 yrds has an energy of 709 ft/lbs vs the 7.62x39 which has 1800 ft.lbs.
Good luck in your choice. I would opt for the .223.