Odds are you'll need a new trackpad/top case depending on the model laptop you have. Pre-unibody is top case, unibody has the trackpad as a separate component.
If it's a pre-unibody MacBook, and parts of the palm rest have broken off along the front edge, then you're in luck. That's subject to a special extended repair program and you can get it replaced free. Has to be horizontal, just along the front edges, nothing on the sides. Otherwise, depending on the model figure on something in the $250US range for non-unibody and probably about half that for unibody. It all depends on the markup of the particular AASP you go to (including Apple retail stores) and what they charge for labor, so keep in mind those are just ballpark estimates to give you an idea what to expect.