Determining the value of a particular board game can be a tricky proposition. It's certainly more of an art than a science.
Many factors need to be taken into consideration, including the age of the game, how rare it is, what condition the box and playing pieces are in, and what market it's being sold in. (What I mean by what market it's being sold in is best illustrated by this example: Fireball Island can sell for less than $5 at a thrift store, but it can go for $50 or more at an online auction.)
How rare a game is generally determines the value of a game more so that its age. Monopoly sets from the 1950s generally are not worth much at all, because there are so many of them available.
Meanwhile, a seller on eBay turned down $700 (in December 1999) for a copy of the 1963 Hasbro Creature from the Black Lagoon game, one that's much less common. (The seller also claimed another copy of the game had sold elsewhere for $1500.) Other interesting prices on eBay: Roy Rogers Rodeo Board Game (1949), $280; Blade Runner Board Game (1982), $305; Hasbro's Merry Milkman (1955), $127. The rare 3M game Jati always fetches top dollar at eBay.
So how do you go about figuring out how much your game is worth?
Price guides are a good place to start, and the best one I've found is Bruce Whitehill's American Boxed Games and Their Makers, 1822-1992. It's an encyclopedic work required for any serious collector. In addition to pricing information, it includes many related topics, including tips on how to store your games and a chapter about the game industry during the 1900s. Another good choice is Board Games: With Price Guide by Desi Scarpone.
Another good way to develop an estimate of your game's value is to search eBay's completed auctions. Doing so will give you listings of prices that were offered for board games at previous auctions, but keep in mind that not every transaction ended in an actual purchase.