Long time casual collector here.
The worth on baseball cards works in mysterious ways, and depending on how you need it to work, it can almost dictate how & what you collect.
Most collections I know of, are built entirely on "sentimental value". Some single cards may be worth a few bucks, but nothing you'll ever get rich from.
The actual cards that carry a monetary value, sell & trade kind of like stocks on the market.
For true "value" on the long term investment side, the older cards, especially pre-50's cards are the place to look for a reasonable guarantee of a return for your money, but condition is a prime factor as there just isn't a whole lot of great ones left.
For the newer cards, you're looking at a short term investment on worth. A hot rookie or variation card that is worth a bundle today, may not be worth anything in 5-10 years, and then just considered common junk even further down the road.
The recent trend by most card companies now, is to "manufacture rareness" by putting out A LOT of limited run cards, most of which are numbered, to increase demand. As long as there is a reasonable interest in the hobby, these cards should always hold some value.
In the end, "worth" is what you're looking to get out of the hobby. 
