When I have donated a qult to be auctioned, I include an appraisal for replacement/insurance value. Of the 4 quilts, I have donated, they have been sold for the replacement value or more. The appraisal gives the organization a starting place and is helpful to them because, it is likely, they do not know the value of a quilt.
Of course, I am talking about a "legit" auction where the people bidding have money and they are purchasing something through the auction as a donation. (not the local Neighborhood type auction).