ummm....what exactly do you mean "Farming FG for gold and win rares"?
if you mean you are only playing them to get the gold from victory, then why not sell the rares (as rares sell for WAY more gold than you win from FGs)?
if you mean you are winning cards you want to KEEP, instead of cards you want to SELL, quit bitching. At least you are winning cards. Anyways, think of it this way: it saves you ~500 electrum, since they usually sell for about 1000, but cost 1500 to upgrade (neglecting the initial card cost)
just my 2 cents
-mad