Ask Ulf from Connect Guesthouse in Patong, Phuket ..
It is all pending on what you need. If you can handly local made medicine you might expect something around 2000 to 5000 tbt, if you need more specific drugs it can run endlessly. I new this from an very lovely Thai friend of mine he need now about 20000 per month for his medicine.
I would suggest to talk to Ulf from Connect Guesthouse in Patong, Phuket. He is a nice, open minded Swedish guy, very much involved in HIV prevention and knows more ore less all answers of your questions. And he is also working at a very good international Hospital Phuket (patients liason). I am pretty sure he can introduce you to some of the advisors there. Since the HIV rate in Phuket is expremely high, the medical team is well up to date with any information needed. This hospital belongs to a chain which is also operating Pattaya-Bangkok and many others in Thailand and abroad. The Connect Guesthouse is one of the best places to stay in Patong if you are looking value for money.
Doctors fee do not vary that much. But at an good hospital the quality of treatment can be better due to better information and education policies.