I think this is a great resource.
One word of caution, though: "The board is worth more than the game" is bound to confuse a lot of collectors and non-collecters alike. If I have a Splatterhouse machine, it should be worth at least as much as the board, because it has the board. Even if, to a buyer, the cabinet and everything else is worth $0.
If we're talking about the inconvenience of dealing with a full-sized machine rather than just the board, I still don't think it should be indicated in the price guide. Too confusing. But again, though, great resource.