Sportsman's Guide has 7.62x39, 500 rounds for $335.97 ($319.17 for club members). You can see it at
http://www.sportsmansguide.com/net/cb/cb.aspx?a=511152
I am a member of the buyers club, and for big purchases my savings has always more than made up for having to get it shipped, esp. because they'll send you $10 coupons on a regular basis (unfortunately you can't combine them, but $10 is $10).