Actually it is rather a lot just to put a car on the rollers and press the loud peddle and then get given a piece of paper. Considering what you actually get for your £35 it's quite a bit indeed, but not so much that people won't pay it, I know, I have myself twice. It isn't worth it though, especially for me and those like me, that have no real need to know exactly how much power one is getting from the car, I find these days I've driven enough different cars of all sorts to guess the HP figure to within about 20 if the car is under 300bhp, margin is wider when they are much higher than that though.
I mostly go because I enjoy the meet up, and to see the different cars.