Sorry to hear about the engine, not many options outside of warranty
I've had my 15 M6 GC for about 6 months now and it's already gone in twice, for replacement of all the injectors and the coils, but I have only paid 50 bucks. And, they are still struggling to figure out why it idles rough. I love how the car feels and drives ... but owning an M6 without extended warranty (not any extended warranty, a top tier "everything covered" extended warranty) is just too big of a financial risk ... so anyone planning on buying a used M6, do yourself a favor and factor in the additional 4-8K for the warranty (depending on the trier, company and your area) so that you don't have to witness something like this. I even went as far as getting pre-paid 2-time-a-year oil changes, because this engine likes to drink the damn oil.
There is a reason these cars go from 140k to 50k in 4 years, they are amazing cars, but just an absolute disaster to own outside the warranty.