We still don't know when the Sprint prepaid versions of the S III will launch but RadioShack may have revealed the price. The white Boost version is now listed on radioshack.com at the price of $429.99. The phone doesn't seem to actually be available yet however. The site is showing as out of stock online and the "find in nearby" button didn't find anything within 50 miles of San Francisco, Chicago, Houston, New York City, LA or Kansas City.
The $429.99 price is a little higher than I expected but it's possible that the price may be different when the phone is officially announced. Even if it does launch at $429.9, Boost and Virgin have frequent sales on all their phones so if you are patient (and watch PrepaidPhoneNews for deal alerts), I expect you will be able to snag an S III for a bit under $400 sometime this summer.
Samsung Galaxy S III Coming to Boost and Virgin Mobile in June