I got an out of the blue offer for a domain I manage.

The offer was $400 for coolassist.com

The domain owner thinks is worth a lot more. How much? he just says I lot more.
He domain is not in use, was purchased for project year(s) ago, but nothing done on that project.
I think he should take the $400.

What do you think it is worth?
Would you try to negotiate higher?
What do you really think it is worth?