Sorry the deal didn't work out. I would feel bad if the seller had no clue of the value of what he had, but your guy is asking at least $10 grand too much for his mule with the 106. Better to walk away and not get frustrated by trying to work out a fair deal with that guy.
Some of the items I would be looking for is of course condition, how complete the mule is, (John Emery of Emery Mule Supply likes to joke that he has over $3,000 in a mule that he bought for $35.00). It is easy to sink thousands in a basket case mule that needs EVERYTHING, and everything that is there either needs to be replaced or rebuilt. You would probably be better off $ wise with a more expensive mule in better condition.
One thing I would caution you about is getting a mule with a wide tire and extended fender modification done by the government on some of the later mules. If you buy one of these mules it will have the turning radius of a bus, and the wide tires are known to wear out steering components much faster than a standard mule.
Good luck in your hunt and if you are unsure on a purchase, you can show pictures here or on G503 mule page and ask other experts for their opinion.