what is the average cost to replace track pad on 15 inch MacBook pro

Repair prices vary so much from place to place aroud the world that it's hard to estimate prices. Contact an Apple retail store or an Apple Authorized Service Provider for an estimate.

Did the trackpad stop working after you spilled something on the computer or dropped it? If so, the trackpad may be OK but some underlying damage is preventing it from working.

Also, a bulging battery can cause a trackpad malfunction in some MBP configurations.

If you have homeowners'/renters' insurance that covers your tech gear (many policies do), put th carrier on notice of an accident immediately. That may be the only monetary help you get. Too often spills do more damage than you can imagine and you could be facing repair costs that would buy you a new entry-level MBP.

On Unibody Macs the trackpad and the keyboard are bonded to the aluminum chassis, called the topcase, and must be replaced as a whole. Besides the cost of the part, the repair is expensive cause of the massive amount of delicate labor involved: the entire guts of the Mac have to be taken out and transferred to the new chassis. I've heard prices ranging from $600 up to $900 or more, depending on location.

In your case, if the trackpad is failing cause of spilled liquid, note that other parts below the trackpack may also be damaged and raise the cost of repair even higher.