What's the bike worth if it was running and well maintained? Probably only about $1,000 for a '90 RMX. Or you could get a comparable '89-'92 KDX that runs well for around $1,000. Personally, I would want a bigger than a $500 discount for the risk and time of taking on a basket case.
If it looks like all the parts are there and in good shape and he has a clear title, I think $200 - $300 would be fair. If you find out there is an unanticipated, expensive repair, you could still part it out and make that much (though you would still be out a bunch of your time).
BTW, parts on old bikes can be very expensive and hard to find - things like a bad CDI unit, worn out shock, shot suspension bearings, etc...can blow your budget and make you wish you had bought a runner.
If the seller thinks that your offer is too low, tell him your goal is a spare bike that runs. Give him your number and tell him you will pay a lot more than once he gets it running. He'll probably call you in a month with a reduced price, and the bike will be in the same shape as it is now.