Nameboy.com offers free appraisals of domain names based on several criteria like number and kind of words, keywords and their use in searches, length of characters, symbol use, etc. It will email it to you. However, I do not know how to base your sales price from the appraisal. It's just for an idea of what it could be worth, not necessarily what it currently is. I think it's probably best to keep it and set up a link to your main domain from it.