average cost per week is £42, average losses are around 3.5lbs per week / 1 stone a month - though you'll likely lose more in the first month.
as for how it is... well, on the one hand it's very easy - because you can't cheat, so there's no bending the rules. also, the fast weightloss is very motivating. On the other hand, the first four days or so are an absolute pig. incredibly hard. and thereafter, you do feel deprived and you will feel sad. it's certainly much easier if you can avoid going out for meals, etc. and some people have faced a lot of criticism from ignorant detractors.
on balance though, this is the only diet that has worked for me. And work it has - in under six months i have lost six stone. i haven't cheated or come off once. I'm on a roll, and i wish i'd done this years ago.