Costs to build an engine??

It costs exactly what he is charging you.

There is really no way to tell you what your mechanic should charge you for the type of build you gave him parts for. No way to know the condition of the parts and the corrections he had to do them. I get new out of the box pistons, rods, crankshafts, rockers, blocks, camshafts cylinder heads and etc in here all the time that take extra hours or sometimes days to make them right. No way to know exactly what agreements existed between you and him as to power, durability or anything else. Besides this is posted in the Race Section, more expensive yet. Sorry, save up and pay.