How to value items not cataloged?

I have been updating my inventory with pricing for each item. All of the items listed in Scott or Bale catalogs are easy to value. But, that leaves a lot of items that are not listed. For example, Palestinian Authority FDC's are not valued anywhere that I can find. Artist folders, bulletins, and other similar items are equally without an easy way to value them. Most of these types of items are not offered for sale individually, only in large lots or collections. The way I am doing this now is to simply value them at what I would be willing to pay for the item if it was offered singly. Is this reasonable?