How much Does it cost to Replace The Battery in the Macbook Air 13 Inch Origianl 2008 Model

Hey so my battery is big and bulky like its blowing up like a baloon slowly so i took it out and i was wondering how mcuh would it cost to get a new one at apple store also im planing to upgrade my system to mountain lion apple says its compatiable it meets just one the line 2GB of Ram 1.6 Intel Core 2 Duo and 80GB Hard Drive which im Upgrading to a SSD OWC lol so what do you think on the battery is it expensive or not