Coin Collectors please help!

I have a 1975 Panama 100 proof gold balboa coin in the original cardboard and vinyl holder with the coa. Everything is in perfect condition, the coin is outstanding! It states that this was the first time the Franklin Mint issued a gold coin. I think that the coin is neat and in perfect condition for it's age, including the holder. My question is, should I sell it or hang onto it--I got it for next to nothing and have seen them go for about $280 or higher--I keep getting kidded that it was minted by the Franklin Mint, but it does say that it is 900/1000 gold minted. any opinions would be appreciated.

Thanks,
Greg58