I think the price for a HUO unmodded Tron Pro is $5k. Because that's what I have, and if I were in the market to sell, that's what I'd price it at.
Any TRON under $4K or even at $4k would likely have a large amount of plays on it.
People are talking about 'vault edition' versions of this game but even at the last run, when I got my game, the availability of new games didn't cause a bunch of $4K Trons to come available. In some examples where the new price and the used price are similar the used price doesn't budge because people don't want those games when they can get new games for just a little more. When I was deciding to buy my game, there was a local TRON that had a large play count that I was considering making an offer on, but when I could get a new one for a reasonable price from a trusted distributor (Thanks PinballMike217 !) , it was a no brainer to get new. Glad I did, because the last run had ramp protectors, etc.