It sounds to me the problem is getting the guy to actually do the work and deliver a completed engine, not necessarily the price of the parts. You will have to have a serious discussion with him to insure your build will be his priority., He already has your money and engine, talk with him and get him to commit to a timetable. Most issues can be worked out if both sides honestly and fully know what the other is expecting.

It's usually better to get a builder who also does the machining but you're already past that point.

One can buy a rebuild kit for a Chev 350 for almost nothing compared to the costs of the same parts for a Pontiac. Just a matter of numbers of SBCs being rebuilt compared to how many Pontiacs. My last build, although not just a simple rebuild, cost me close to $11,000 just in parts I did all the work myself. A bearing and ring job is one thing but once you get into replacing all the internals the costs rise fast.