Kimi - my Mum, my DD and I went in December and if I remember correctly it was about £30 for the three of us. A woman who was with her friend and her son came out because the son had got scared just before the bit where you present your tickets, and sold us their tickets while we were waiting in the queue. It was great because we were still a way back and the tickets gave us access to the 'fast-track entry'
As I say, this was in December so prices may have changed since then, but I would say allow at least £10 per person (plus extra if you want the photos they take inside)
Hope this helps,
FBB