LR - Not sure what you mean by 'still in the original plastic case', but I can tell you about the stamp. It is a definitive from the Great American Series of 1980-1985. If the stamps are unused and unhinged (aren't we all sometimes?) the catalogue value is 40 cents, but typically might be found at face amount plus 5 or so cents; suggest you use them for postage. If they are attached to an illustrated cover (envelope) they might be worth more...maybe a buck at retail.

The sad fact of US stamp collecting is that most stamps of the last 50 or so years do not really appreciate much at all. I stopped collecting US about the time that stamp was issued.