If its just the battery its a straight forward job. I have changed the battery on one our fobs, it takes a coin type battery easily available on the net. Only thing to be aware of is, in the re-assembly be careful re-screwing the tiny philips screw that holds the fob case halfs together, its easy to cross thread and strip the plastic thread.
No idea ref cost of a complete replacement complete fob, but I know just the key fob case less the electronics is over £50.00
On one of the sets of ignition keys for our car we have serial code info for the transponder, but not sure it that is just for the ignition key or is also for the door key fob.