[adventured]

If it's not already listed for sale somewhere for ~$50, it's very unlikely you're going to get it for a relatively low figure.

Once someone gets contacted about their domain, their estimation of the domain's worth automatically jumps. So if they're thinking it's not all that valuable, your contact will make them think it's worth something. Even a layperson will think it's worth hundreds or thousands of dollars.

[JoblessWonder]

I'd also check out the whois info and see how long it has been registered. Assuming it is a .com address multiply the number of years by $10 to get an idea of how much they have been paying to keep it registered.

They are going to want to at least recoup their cost in a sale (sunk cost fallacy?) so I'd offer them at least that plus 10% so they aren't insulted.