How do I find out a fair price for my Magic the Gathering cards?
If you want to know what you can actually sell individual cards for, I think you can't do better than ebay. If you look at price indices like Scrye used to have, you run into the problem that what a shop can sell a card for is typically much higher than what you, a private individual, can achieve.

Bear in mind as well that prices for individual cards are much higher than buying in bulk. In other words, you might work out that your collection is worth thousands of dollars, if you were to sell each rare individually. But if you tried to sell the same thing to a shop, or in a single lot on ebay, it would sell for only a small fraction of this.

The only other caveat is if you have many extremely rare old cards from the first days of Magic. In this case, the wear and tear of the card has a very large impact on the potential price. Again, ebay will give you an excellent guide to how much these can vary (try doing a search for "Black Lotus" for example).