The easy way is just to base it off of a price that would make you happy. Remember transferring is going to take a bit of your time.
1) Think of a price that would make you excited to sell it, then add 25%.
2) Send that back to him and see what he says. Something along the lines of
Dear Mr. Domain Man,
Thanks for getting in touch. I’d definitely consider selling thisdomain.com, [and do | but don’t] need help with otherdomain.com, thanks for offering.
If I were to sell I’d want to get a value of about [magic number + 25%]. (discuss how you’d want to figure out the value of development time if you’re interested in that)
Does that sound reasonable? Let me know your thoughts and we’ll go from there.
Thanks again for getting in touch,
Rexpresso and the whole Saving the World Crew
(see, you discussed with “your people”)
3) If you never hear from him again, he was hoping to get it a lot cheaper. If he make a counter offer, hopefully it’s close to your original number and you can accept.
4) Save world with new found funds.
You can spend all day doing research on it, but you may be worlds apart on price and it would just be a waste of time. If he’s offering a domain in exchange, he’s probably got quite a few and has a ballpark in mind already.
If you want to research it, I’d try to find out as much as I could about his business, does he put up parking pages and collect off the ads? Is he likely to develop a site for the domain? Is the development time he’s offering you worth anything? Can you find examples of work he’s done for others?
The basics are going to be the same whether you spend a week on this or 10 minutes though, so I’d just shoot him a quick email Monday with your happy number and go from there.
Good luck.