Used MINI H/K amplifiers show up on eBay pretty frequently. There's one on there now that's up to $207, and three more that completed in the last few weeks for $300-$450.

That's much higher than usual, so there may have been some bidding wars involved. I've often seen them go for as little as $100.

I believe the H/K amp is different between the coupe and the convertible, but I also believe that the amp is the same for all first-gen coupes, so as long as you don't buy one that came from a convertible, you should be fine.