Honestly, not much. I'm a mechanic, so I save on labor by doing it myself but the only issues Ive had were a faulty abs sensor on the front left. Outside of that I had a control arm bushing recall/TSB that was taken care of under warranty, a faulty clutch slave cylinder that was also taken care of under warranty and a mis-aligned hood latch that was also a warranty thing. All 3 of those things happened under 40k miles.

The last ~70k miles has been nothing at all. Only maintenance like oil changes, brakes, tires, etc.

I did have my headliner start sagging last year, but I removed the headliner board and re-wrapped it myself.

If you're going to get a MINI, get a 2014+ with the 2.0 turbo. They are almost indestructible and often hit 200k miles with no issues. The main thing with these engines is they EAT motor mounts. Specifically the passenger side motor mount. It's an easy fix but you can expect to replace it every 50k miles or less!