Howdy Pat,
You have some pretty good responses already, but here's my ten cents. No matter how many T-206s or Cracker Jack were made, and no matter how many 1952 or 1959 Topps were cranked out, it comes down to how many have survived today. Even if they made a ka-zillion T-205s, and only 50,000 are around today, that is just part of what matters, though.
Someone mentioned the key phrase--supply and demand. The supply of something might be rare as a hundred-thousand dollar bill, but if it's ugly, or depicts common players, most collectors do not work up much of a sweat over them, except the "type-kwandos"--those guys LOVE that rare stuff, and they really do get their kicks with them, which is cool. I like their passion.
Demand is really a very crucial factor, because it plays out in more ways than a lot of collectors realize. For instance, supply might seem abundant, but if it is really popular and beautiful, and a lot of collectors admire that particular set or player, then the available supply is diminished severely. Those collectors could easily savor those cards for 10 - 20 - 30 years. That is a long time. (T-206 definitely comes to mind) You best know that when a desirable piece hits the market, a lot of collectors are now factoring in "frequency of availability". That is not official, just made it up, but regardless, it describes the importance of knowing what you are facing when you decide to collect a very popular set or player. Demand breeds higher prices, of course, particularly for the upper echelon and especially the highest graded specimens.
Wrapping it up Pat, the guys who said to go to eBay are right on for showing you what is readily available now. There is post-war galore for most of the gum cards. The pre-war picture I do not know, but suspect it varies. There are post-war regional cards that are brutal as all get out. Please don't go down that road--I don't want the competition!
The auction catalogs will convey what costlier and often more desirable items are available.
Now finally, one of the best ways of knowing what is "out there" is to check the population reports of PSA (Professional Sports Authenticator) and SGC (Sports Card Guaranty). You have to be a member of PSA to get the full dope on their pop reports, but it's well worth it if you are going to seriously persue certain sets/players. SGC, bless their hearts, offer their pop reports for free. Granted, there is controversy over what is still out there ungraded, but the pop reports, since they are now 15 years old, offer pretty accurate patterns of scarcity and theoretical availability.
Now that I have bored you to tears ol' Pat, collect what you love, usually the money will follow when you sell. -Bri (ol' Mike)