Okasha Mohamed Ahmed

Guys..i've got an urgent question cuz am about to take a decision i know nothing about..
I got 1908 St. Gaudens $20 in perfect condition.. am not even a coin collector but i had way ling time ago from my father before he passed away.. few days ago a friend of mine called me n said he met a man who's very intersting buying it..i met him n he offered around 80,000 thousand dollars equally to his egyptian pounds offer..
I found some similar coins of mine on the auctions on web and ebay have been sold for more than half million dollar!!
So my question here is it worth 80 thousand dollars or if it worth more and if more how can i sell it online and for aprox. How much??
Sorry for taking too long but here in egypt its quiet rare to find collectors who really understand values..thanks alot