Just for the domain name, I also think the mid $XXX is about right, so the offer sounds good. It is a great domain name, but unless you plan to develop it yourself, you might as well make some quick cash to invest in your own business (or to buy more great domain names to sell).
I don't think it's necessarily going to appreciate in value much higher if you hang on to it. The only suggestion I would offer is that it might go for a bit higher if you were selling it just before school starts in the fall, but that's just a guess. I usually prefer to take the cash when it's offered rather than gamble on it perhaps going higher, 'cause it could also go lower
