dominic77's detox centre.
Location: Any cheap bungalow on a quiet beach.
Materials required: Reading books, iPod, self-discipline.
Schedule: Get up early. Drink lots and lots of water. Eat plenty of fruit and vegetables. Minimise your consumption of processed food and refined starches like white rice. Obviously, don't drink any alcohol or smoke. Do lots of exercise - run along the beach, swim in the sea, do push ups and crunches in your bungalow. Go to bed early, and make sure you get at least 9-10 hours sleep a night. Repeat daily until you feel suitably "detoxed".
Costs: 200-500 baht for the bungalow (per night). Basic food and drink. Otherwise, it's free!
If you lack the willpower to do it yourself, there are plenty of places in Thailand willing to part you from plenty of money for their "professional" detox, as you have discovered.