It looks as if that stamp is a Van Dam OLT6 Revenue Stamp. You are correct that it was issued but never used, therefore these stamps are more accurately considered "remainders".
Values of such items are very subjective depending on how much a collector is willing to pay and a seller is willing to sell the stamp for. The Eric Jackson Revenue site has this stamp posted on the retail level for $17.50, so one can assume that if you were to offer it on the secondary market, it would sell for considerably less:
http://www.ericjackson.com/catalog/..._code_052334