I don't know if they still do it...but when I worked for Tilt, they would put all their games on sale around Christmas. All the machines would have price tags on them with the prices. The older games would go for $250ish and up depending on the game.
When Super Street Fighter II had just come out, we had converted an old game into SSFII and that week someone came in and wanted to buy it. It went for $3995 and since I was the manager I got a pretty nice commission for that one. I also sold several other games that year.
I don't remember if there were any Tetris machines in the company at that time. They probably were all converted into something more profitable. It might be worth it to stop by and check it out though.
The same company also owns The Gold Mine and possibly the Red Barron shops in case there's any of those in your area. Also, sometimes the small independent shops will put their games up for sale.
I think it would be pretty cool to have a coin-op Moonwalker machine. I ended up converting mine and I don't remember what it became.
