Developed value could be anything, depending on how "developed" the site becomes. If you have a serious buyer more than reg fee, I'd sell it ASAP before buyer changes their mind. Just curious as you mentioned a lot higher than reg fee, did you by any chance receive an email sent to you by a "buyer" that is wanting to purchase your name, but requires an appraisal 1st? If so, he's an appraisal scammer.
I am agree with htmlindex, If you have got offer in email saying that we would buy your domain for $xxxx but we first need appraisal from "blahblahblah" site then it must be a scam.