How much will it cost to replace a trackpad

We are all other users here and are not privy to their pricing; however, it also very much depends on your model, if you have Applecare + . Best thing to do is to take it to your nearest Apple store or start a support request (upper right corner on this page) with a hardware problem. You should wind up with an option to make a Genius Bar appointment. After they look at it, they will be able to tell you.

The part is not that expensive for this model. Labor is likely to cost more than the part. The repair will likely be cheaper if you the AASP or Apple performs the repair on the premises instead of mailing the laptop out to Apple.