The first day cover featuring QEII Royal Visit in 1977 with the silver dollar is worth as much as a buyer is willing to pay. There were two thousand covers with silver dollar prepared and marketed in 1977.
I have several identical covers in stock, one in fact listed on eBay, and have not (yet) found a buyer willing to pay at least $25.00 which I consider a "bargain".
To find its true value, you can start an auction at $0.01 and see how high it goes. That would be its "true" value. Are you willing to risk selling it for only a few dollars if few interested buyers show up?