The ones with the "gold stamp" are not highly sought. Even when listed for a couple of dollars on
ebay, they do not sell, certainly not to knowledgeable collectors. FDC's with plate blocks from the period you mention could be worth a few dollars in some cases. Depends on the stamp, if cacheted, and the designer of the cachet. Common Artcaft or Artmaster would not be worth a lot, but Fleugel and some others are worth something. Just too many variables to make many blanket statements without seeing specific items.