The store I was in today had boxes of 1,400 cases of 7.62x39mm for $90. There were 20 round boxes for $3.50. So, that should be an indication of approximately what you should expect to pay.
boxes of 1,400 cases of 7.62x39mm That should have read: cases of 1,400 rounds of 7.62x39mm.
That's what I typed; my fingers just output the wrong letters. :-P