As Pawain indicated, a lot depends on why someone is buying it. My best guess is 25-50 mil based on recent observations. I recently purchased one for 60 mil, but since I now have one I won't go that high again. The color can also make a difference, the jet black ones are usually preferred. My suggestion is to run an auction. Show screen shots so people can see what they look like. Determine a reserve price that you won't sell them for less, and choose an end date for the auction. Many interested parties won't give you an offer if the sale has no definite closing date.
I assume that they are both 2 slot. Just checking since the one nightmare I had in my stable that was definitely pre-patch/legacy, was a 3 slot. Yay me!