I'm afraid I don't know how much it costs these days, but some 15 years ago I did it and it worked a treat, but then when eating only 600 calories a day, it would!
It is actually very dangerous I feel now that I know more about it.
Slimfast works well, along the same principle, but it incorporates normal, low calorie meals of normal food into it's plan. You only have slimfast for two of the three meals in the day. It actually isn't so bad. At least it isn't as tasteless as the Cambridge diet. I actually like the coffee Slimfast as a normal milkshake.
Think very carefully before you commit to the Cambridge diet. Consult with your doctor. It is after all, a crash diet, and these are never good.
I wish you all the best.