It's been years since I bought any JAMMA boards - my Taito F3 board collection cost about 40 quid or so each at the start of the decade, and are easily three or four times that much now. The most valuable boards I ever picked up, as part of an arcade job lot that got me started, were an original Namco System 1 Splatterhouse and a four-player Simpsons. God knows what the Splatterhouse in particular goes for now.