Yep, the boards really DO go that high.
The reason is that Out Run was Mega popular and the games saw extremely long times on location, thus almost none of them ever got converted. Heck, some of them are STILL on location (I bought one a few years ago that had a current tax sticker on it).
Since no Out Runs ever got converted (to anything other than Turbo Outrun, which is not a whole new board), that means that you have to kill a machine to get a board. Making matters worse is the fact that Turbo Outrun sucks, and a lot of owners want to go back to regular.
Add that to the fact that some locations are STILL running an Out Run machine, and that is where the price comes from.