The cost of Phentermine treatment usually escalates when combined with other weight loss programs or treatments. Some weight loss clinics recommend additional intake of supplements to boost the weight loss process. B-12 injections and HCG injections are common addition to Phentermine treatment. This is where the most of your money goes. An HCG-Phentermine combo will cost a patient $50-$100. However, in some clinics, it ranges from $400-$1,500 depending on the duration of the treatment. B-12 injection will also cost $12-$20 per injection.