99 times out of 100, on our cars it's the beginning of a blown head gasket. Other problems could be a chipped valve, bent valve, broken or worn valve spring, damaged valve guide or valve seat, damaged piston ring, cylinder wall damage, etc.
Price to fix it could vary anywhere from a couple of hundred dollars to considerably more. For example, if you've got a bent valve, the only thing that might make sense is to rebuild the head, or swap it for a known good used head with another member who's getting a head built for performance.
If all of this sounds like more than you are willing to deal with, you might want to rethink your choice of a 16 to 22 year old sports car. Admittedly, the Supra was Toyota's absolute flagship car at the time. It's built better than almost everything out there. But these cars are old, and sometimes need a considerable amount of TLC to be 100%.
