Engine Mechanical problem
1991 BMW 325 4 cyl All Wheel Drive Manual

My car had been leaking oil then coolant. The mechanic replaced the water pump, but the problem got worse. He now says it is likely the head gasket . How much would it be to replace this? The car is in decent shape otherwise. Is it worth it. How much could I selll the car for also?

It may be worth it for your mechanic, only you can say if it is worth it to you. R&R a head gasket and checking for a cracked head could easily exceed the value of the car if you play the worst case scenario.