I would pay $2500 for a brand new Seawitch. Probably even $3000.
No multiball. Keep the rules as they are now. In fact instead of porting the source code to some bespoke platform just buy the existing replacement boards from like rottendog in bulk.
I still don't think it's possible. Someone above just outlined a $2500 BOM and that's not including labor. I feel like labor is the big cost here and that the BOM for a Seawitch is not significantly different than a low coil count pro like AC/DC. The difference is expectation: Seawitch looks simpler therefore should cost less in collectors minds but given a fixed labor cost it really isn't cheaper