If it was in good shape I wouldn't be above paying $300+ for one if it was near me and I could test it beforehand.
Sight unseen I would still offer $100 for one. They really are THAT good.
On the technical specification side these things are very impressive.
High resolution, multi-sync, in a case, vga connection, s-video connection, incredible picture, built in speakers, etc.
The only drawback is no warranty, no support from the company, and less knowledge on repairs or sourcing replacement parts here on the board. Also you might have to mod a cab a little bit to fit one.
I would say offer $50 up front. Oftentimes people can get these for around that price or less on craigslist or ebay. Feel out your buyer and figure out if he knows what he's selling.
If you don't want to mess with an arcade monitor and still want arcade resolutions for your cab this is the best possible choice.