The key decision to make when buying a pair of binoculars is magnification versus size.
You'll notice binoculars described by two numbers - for example, 8 x 40. The first number describes how many times normal size the binoculars magnify the subject you're observing; the second number indicates the diameter, in millimeters, of the so-called "objective" lenses; that is, the lenses on the opposite end from where you place your eyes. Since high magnification binoculars with large diameter lenses tend to be heavy, and low magnification ones have lenses too small to admit enough light, most birders use binoculars with 7x or 8x magnification and lenses of 30 to 50 millimeters in diameter.
Other things to look for are a relatively large field of view, making it easier to scan for birds and follow a bird in flight, and image stabilization technology which prevents image "shake". This feature is becoming quite common on even lower priced models.
What should you pay? Medium-priced binoculars can be found at between $100 and $250, although you can find a pair for well under $100 if you're willing to compromise on quality. As with everything else, you get what you pay for!