It is not hard at all to find extremely clean rooms in good tourist areas for around 10-20 dollars a night. If you go with a fan room you get a few dollars off but my experiences would recommend getting a room with AC. Massages are usually about 5-6 dollars but price can vary but a couple bucks depending on the location and the type of massage (not talking about the happy ones
), As for meals, it's about 60-100 baht (2-3 dollars) for a typical Thai dish with steamed rice. Drinks are generally about 50 cents to 1 dollar if you aren't drinking beer.
For a 2-3 week trip you can definitely do it for much cheaper then 2500 a person but if you are someone who likes a very structured vacation this wouldn't be too bad at all.