I just had my keyboard replacement last week at the apple store due to a faulty spacebar. Luckily for me it was still under warranty (new MBP '10).
Total cost for changing the keyboard was £238.52 including VAT (17.5%). Total time for repair was about 5 days. Breakdown of price is: -
£162.15 for topcase with keyboard
£76.37 for hardware repair (assume this is cost of labour)
I'm sure you can get this cheaper using a 3rd party service, but this was my experience was through the apple shop.