I really don't upgrade except for a few sets or items, but I do look for cards from my childhood collecting era (1970-1981) at the National. I mostly go through deep discount boxes and grab as many different nice, sharp centered cards as I can find. There are tons of them available so the only trick is finding them clean, sharp and centered enough to satisfy my tastes. These are examples of the stuff I look for and spend a buck each or so:
I don't care about PSA holdering; these go in my albums. Visual appeal is paramount for me so a tiny technical issue is not a consideration. And as long as I'm only spending a buck or two each, I can buy one that's pretty nice, like the 72 Allen, and see if it improves my album or not, and if not, no biggie.
At this point I am down mostly to centering upgrades on the 1970s cards.