Short answer, under $1000.

Long answer. You can get an s19 95t for around $500, then the water blocks are the next biggest hurdle. You'll generally spend about $80 per board, but they're frequently only sold in sets of three. The water cooling gear is off the shelf PC cooling hardware and it goes together really easy. There are cost savings if you build more than one (I'm doing three, cause ive got three water blocks now, obviously 🤣) or do group buys. The case I'm using I built as a development box for this purpose six years ago. Its space heater life has housed GPU's, s9's, and now an s19. This iteration is the best so far.