I would offer $350 and explain to him that you have other costs involved and then there is the mystery aspect to it. If he indicates some specifics, then I would go higher if it seems safe. If he rejects, I would say 400 and then stick to it. You would easily have 100 into van rental, gas, time, etc. - and $500 total investment seems very fair to me even without knowing what it involves for sure.
Think about it - even with what you see, it would not be hard to sell off for $500. Then consider that it would just take one discovery to send it to the moon! Could be some gem cymbals in that cymbal bag. Snare case could have a gem in there as well. So many possibilities. $500 is a great deal from you side - as even an MIJ kit could sell off for $200 easy. Not going to be hard to replace a $500 investment at all. We haven't even started to talk hardware.....there is probably $500 in hardware alone there.
I wish I had that deal in front of me!
I would even have a hard time sleeping until I got all that home.