How prices are determined
The most important thing to remember about a price guide is that it is only a guide. A set is really only "worth" what you are willing to pay for it. Prices listed here are prices I've seen these sets offered for and/or sold. That means they can be sold for much more or much less. I'm not offering to buy or sell anything at these prices-- they are basically estimates. And remember, "retail" is what the established dealer usually sells a set for. He buys it for 33% - 50% less to afford himself an advertising, overhead and profit margin.
Condition is a very important factor. Ex sets are often 1/3 to 1/2 the price of the same set in strong Nrmt to Mint condition (depending on age). Newer sets (1980s and up) are almost always priced in Near Mint (Nrmt) to mint condition. Earlier sets are often priced in Excellent condition (Ex).
Ex is a card with very minor corner dings or wear on the corners, but not rounded or creased. A very collectible card.
Nrmt allows very slight wear on a corner or two, but basically a new looking card. Mint is a new card, no problems, no off centered images, dinged corners, etc., (or else downgrade it to Nrmt.)
Remember: The more expensive a card set is, the harder it is to sell at its retail price. That's because most folks who can afford it already have it, and those who don't, want to buy it at a bargain. In other words, the potential market is smaller because fewer people are rich enough to pay big premiums for high grade cards.