I'm a PT and the gym I work at charges me out at £33 an hour. Its pretty common practice to have to block book a set of 10 at any major health club. Its not a money making scheme, but its what you realistically need before you start to see results, i.e about 10 weeks or so. You might otherwise get people paying for one or two and then dropping out as they haven't brought in to achieve the results as such.
I'm not going to enter the whole its not worth it debate, as I wore my fingers out last time around. Put it this way though, £33 a week is what some people might spend on newspapers and coffee in the morning. Its what you might spend on CD's, a few pints down the pub on the weekend, a couple of pizzas. Its up to you to place the value on a good or service. I've had some fantastic results with clients. Whether they would have achieved the same reading things on the internet, who knows.
Oh yeah, and before you start thinking we are all minted, you wanna see the cut the gym takes out of the £33 a session.