When I built a 235 for my 52 I spent about $1000 for machine work and parts, did the assembly myself. That was in 2000. Rebuilding a 216 would have been about the same. Higher now due to inflation.
Ease of dropping in a 235 depends on the year the engine was built. The 54-55 models need the least work, about the only things you have to change are the accelerator linkage and the exhaust pipe. Later models also need a short shaft water pump and the front motor mount bracket added.
Ralphy is right, the problem will be finding a running 235.
