[raulus]

Interesting thought exercise.

Part of the fun is that by definition, a piece would have to be pretty well unknown or undiscovered. Because if there are only a few of them and they're well known, then the price usually starts to rise above your target range. I suppose the exception to that rule is if a piece is well known, but for whatever reason no one wants them, either because they don't care, or because they're just hideously repellent.

I suppose the other factor is whether a grader will slab it. History has shown that there are plenty of overlooked issues that aren't in slabs, but once a grader starts slabbing them (looking at you, PSA), then the demand for the item (and therefore the price) start to jump in a hurry.

I know that the OP implied that this is just for pre-war stuff, but being a mostly postwar collector, my mind naturally gravitated to a lot of the food issues, which are often difficult to find yet still relatively inexpensive. Outside of maybe the superstars who trade a little more frequently, any given piece from a set might only trade once a year or less. I'm thinking of stuff like Post cereal, Jello, Bazooka, Stahl Meyer, and Briggs Meats, which loosely fit your criteria, except that the HOFers in high grades will trade well above your price range. I'm sure that we could add to that list without having to dig around very much.

If these issues don't tickle your rarity fetish, then you could always shift gears a little to maybe stuff like complete Jello boxes and complete Bazooka boxes, relatively few of which survived intact. While the HOFers will still command big dollars (particularly in higher grades), the lesser known players tend to trade for a price that isn't far from your range.