It'll likely be £10 if you get the stamps from your friendly Travel Agent rep.
It'll be the official fee of $15 if you buy the stamps from the official window - as it says on the sign under the roof advising on current visa fees...
.. just depends which Q you decide to join and how much hassle you can take from the reps
Whatever you do, make sure you have the exact amount. Chances of getting change are usually slim.
Have fun!