Zone91: you are talking about what we call a "back run". This can be more expensive than collecting the whole set. Some backs are quite rare (read: expensive) and to find a rare back on a star player will be very expensive. There is a Cobb with a one-of-a-kind Brown Lenox back in a Goodwinandco.com auction right now. The last I checked the bidding was over $22,000.
I'm not trying to discourage you. I just want you to know how expensive your goal can be.