It's mostly that printing proxies on my home printer also works and is really cheap :) I am working on that blue cube LadyMapi suggested though, and for that I would need a bunch of original duals and fetches...
So, what I'm really interested in is printing high quality nonfoil customs, but I'm guessing that's a lot harder than getting foils of existing cards :)
Wait... I have that deja vu feeling...
Im curious, why do you want to know? I mean, isn't this kinda known information? Ancestral Recall, Time Walk, the original Moxen, Black Lotus, Sol Ring, the best conspiracies, Lutri, superior mana fixing.