What kind of a Domain Name is that?
A generic Domain with a number of exact searches per month?
A brandable Domain?
A person with 800 Domains probably knows more or less about the value of domains. Unless its a crap name, you should offer the approx. value of the domain (if its generic name). If its brandable, it all depends on the negotiation strategy, etc. He could end up deleting it and you could backorder it or even hand reg it later. Or he could keep it for years and don't accept your lowball offers. It all depends what kind of a domainer you are dealing with.
If this was a brandable domain and if this was me, i would simply email him and write "i can offer you 100 dollars for this name" and see the reaction.