Probably not much, but here's the rub... you will probably need the whole SLK32 AMG to make sure you get all the parts needed... ECU, any additional bits and pieces, mounts, intercooler, radiator (probably different capacity), brakes (might as well, they should fit).

The devil will be in the details about what you WILL need as opposed to what you WANT.

If you have garage/shop space and can do the swap in your 'spare' time, this would make the prospect easier. BUT, for the amount of time and energy you would put into it, you may want to simply trade in or purchase one of the lonely SRT6 Crossfires out there for around $25k.