Presuming, of course, that you don't sell the 7.62x39 to fund the new 5.45 rifle.so you'd need to shoot 6K rounds to break even.
Well, that's hard to say. What's the gun for?
PS- if you are going to buy 5.45 bulk (and I mean bulk) check out Century Arms. If you buy $500 or more there is free shipping- plus the overall discount is huge ($40 off per case of 1080):