There is a big difference between the price of a coin, and the value of a coin. Although you often see these words used interchangeably, it is important that you understand the different concepts represented by each.
The "Price" of a Coin is How Much it Would Cost You to Buy it From a Dealer
This is pretty straightforward. The "price" of a coin is merely the amount that it would sell for on the open market, otherwise known as its "retail price." Coin prices are set by many different factors, including the type and grade of the coin, its rarity and desirability, and to some extent its availability in the marketplace.
The most frequently used price guide to U.S. coins is the Red Book.
The "Value" of a Coin is How Much You Can Sell it for Today
Here's where it gets a little complicated. When you want to establish what your coin collection is worth today if you wanted to sell it, you are establishing its value. The amount you can sell your coins for (its "value") is considerably less than its "price" if you had to replace them. Dealers need to make a profit to stay in business, so when you go to sell your collection, you're not going to get those nice, high Red Book prices. The Red Book prices are retail amounts.
Consider the Blue Book
There is another book, known as the Blue Book, (formally titled "Handbook of United States Coins"), which is the most widely used guide to wholesale coin values. These are the values a coin dealer will offer to pay you for your collection. They typically run about half of what the coins retail for.
Coins which derive most of their value from bullion (such as common-date American Eagles and Double Eagles) will get you more (75% to 85% or so) because most of their value is based on the gold itself, rather than the rarity of the coin.
Appraising Your Collection for Insurance Purposes
The one time when it is correct to use the "Price" metric to determine what your collection is worth, is when you are establishing its value for insurance purposes.
In this case, you want to insure the replacement cost of your coins. Since you'd have to pay the Red Book (retail) price to replace them, this is the coin value listing you should use.
Always be Realistic About Prices and Values
There is nothing more satisfying to a collector than to pluck a coin worth $100 in the Red Book out a dealer's $10 pick bin. And in this case, you've probably done very well, because it's likely the dealer overlooked something here. But the more typical case is finding lots of $20 Red Book priced coins the $10 bin. This is because the dealer is probably overstocked in this material, and would be happy to get his cash back to make more marketable purchases. Be careful that you don't get carried away thinking you're getting bargains in cases like this, because the amount you can sell the coin for, its value to you, is about what you paid for it. In other words, don't deceive yourself into thinking that the value of a given coin is equivalent to the price you paid for it.
Consider getting yourself a copy of the Blue Book so that you can get a realistic handle on how much you can actually sell your coin collection for today if you really needed to. Compare Prices for the Blue Blook.