I'm not sure on the value - size, power, condition manufacturer etc. all play a part. Brand new, known brand ones seem to be able to command several hundred pounds on ebay these days, others, much less.
The main factor is whether it actually works or not. The alignment and condition of the mirrors (one 100% reflective, the other, the beam exit end, about 99%) is critical, They need to be spot on for the tube to go into laser mode. Also assuming that the seals are intact. If you have no evidence that it is functional, it becomes a pretty ornament in price terms.
Generating those sort of DC voltages is a lot easier than it used to be. Normally comprising a flyback inverter followed by an ordinary voltge multiplier of a few stages and the all important series ballast resistor (covered on the website). That sort of setup can be run from a low voltage supply rather than directly from the mains (the output can still bite though!).