the best way to "trade up" is to sell the game then use that money with the extra cash you have accumulated in that time frame. This is precisely how I got my Stern Star Trek. No way was I gonna drop that much coin on a game at one time but I slowly bought/sold/traded up and eventually sold a Batman Forever and with that cash traded it with a Bride of Pinbot for my ST.
Just dont pigeon hole yourself where you want to get X game for your Y game as that really limits your opportunities