Its crazy lately, CPS3 boards have started going for ridiculous sums of money. A couple of years ago a full board with a game was about £350, they have gotten close to double that now for what I've seen in some instances.
Just gotta vote with your wallet and (sadly) be patient if you want better prices. Multiple CPS3 boards have been posted here the past couple days, and surprisingly none have immediately sold. Just last year I picked up a full SIMM board with a working drive for 550, so the uptick has definitely been around regardless of availability it seems. If the price isn't right...just gotta let it sit.