Finding prices for your cards.
So now that you have your cards sorted and have a decent idea of their quality, how do you come up with prices? The answer is, check the internet. There are several places you can check, listed here, each with a small review.
StarCityGames - This site is a good place to get a decent idea of card prices, by looking up what they would cost you to buy. Each card and each edition, as well as foil or not, along with the card quality is listed with price. They also have a buylist, although at the date of this writing it's down for improvements.
BlackBorder - This site has more of an article-style homepage, and writes posts about Magic cards, decks, tournaments, and expansions. It's sell list tracks prices from several different sellers, and it has a more comprehensive buy list than StarCity Games'. Their buylist is also sorted by edition, which is quite helpful.
Ebay - It may seem obvious, but many people buy and sell cards on Ebay, and more often than not the current prices on Ebay will be reflected in the prices on other sites. The trouble with Ebay, of course, is that it's an auction format. You could get cards much cheaper, or you could be outbid a dozen times and end up paying more than the card value. Ebay also tends to be a good place to sell collections of cards in bulk, so if you don't care about individual card value and simply want to offload a collection, this is your place.
Troll and Toad - Listed above because of their comprehensive card quality guide , Troll and Toad also has a decent buy list and is similar to StarCityGames in many respects.
Bidwicket - This site is the ultimate in card pricing guides. They have a comprehensive card search, with all the Gatherer errata text and card pictures. Each card has each edition tracked separately, and Bidwicket tracks dozens, if not hundreds of different sellers to show you all of the prices on all of the sites, ebay sellers, and anywhere in between.
So, even if no comprehensive Price Guide for Magic the Gathering can exist, these sites are as close as you can get.
Is Magic not your cup of tea? Read about the Nudity in Heavy Rain instead, or check out this guide to Lexulous scoring.