If you want a cheap backup device get a Gameshark Pro v3 and get the prog for it at the Game Shark Code Creators Club site. I don't think the latest version is there, but a late enough one for backing things up is.
NOTE: Gameshark Pro can dump any cart size so far as I know, I dumped all of mine. It doesn't store anything on it, it dumps straight to a file on your computer, so I don't see why there would be any limit. I do know it is a bit slow, but I don't know how it compares to other devices. $30 new, much less used... not a bad price, hehe. Don't but it used though, they tend to wear out sometimes, and the savings isn't worth the potential pain in the ass.