My reman 6.0 was $2500. Plus another $400 because I didn’t have a core. Granted , pre-Covid price. I spent about another $2500 on externals( intake, coils, bracketry, alt, p/s, a/c, water pump, front brackets, valve covers, belts etc.), plus $1500 on a reman 4L80E. I did my own harness, now I did most everything else myself. I would estimate by the time I was done with everything I’m at $9000-$10,000. All the little stuff adds up so fast. Cooling system mods, hoses, brackets, hardware…
Figure in the design, welding, and infinite plethora of mods to make it all play. It’s not for the faint of heart if you’re doing it yourself.