Leap Wireless owns Cricket and just got access to the iPhone. The iPhone 4S will be sold for $500 for the 16GB model and the iPhone 4 will be available for $400. The key is that there is no contract.
Tidbit: Leap holds 209 licenses for PCS, Lower 700 MHz and AWS spectrum. Leap has been in talks with AT&T and VZ about spectrum swaps and is swapping spectrum with T-Mobile.
I would check first to see if they are in your area.
It's cheaper to buy it prepaid than postpaid.