More than the gun. That's if you want to make it bulletproof.
Basically, if you intend on running lipo in it. count on replacing all of the internals. And if you're clueless about mechboxes, a complete mechbox is the best option.
However, the down side is that for the price of a Kraken plus all the replacement parts, you could get a better quality gun that wouldn't need all its internals replaced.