The most you should pay is the $10 or so to get the same domain name but with a different TLD.
Once you're up and running and generating revenue, then you can worry about paying for the .com.
(The price that the "domainer" is asking isn't going to come down. It comes him practically nothing (<$10/year) to hold onto the domain name -- which he'll definitely do now that he knows someone is interested -- since he is your only option if you really want it.)