I think I would start out by asking what they are willing to offer. If it doesn't seem like a fair trade then I'd start haggling from there. I wouldn't even consider doing it for a normal!

Will you have to maintain and update the site after it goes live or just design and build it?

When I sold my website, I had been contacted by somebody after the sale who was also interested. She made me an offer to build her a duplicate site which was nearly double of what I sold the site for in the first place. I got a fair price for my site but I guess it was just worth more to her than that. My point is, the site may be worth more to them than you think it is and if you sell yourself short, you could be missing out, like I did.