The 7.62x25 Tokarev round is a great round that used to be very inexpensive. It really is the caliber that people wish the 5.7mm was.
The Soviets used the Tokarev round in their submachine guns in WWII.
http://www.youtube.com/watch?v=FTg-TgEpUm0
$20 per 70 round box. But, the round is getting a little bit hard to find from what I hear.