I don't know any agencies but if you want to do it yourself and save a lot of money it's not hard to do. I paid for a 6 month training package with Phuket King for 15,000 baht and then applied. They have all the correct documents.
for once I will agree with you. 27k is a fair price and that gym Fitness Hero in Patong he got the price from is actually a really nice gym. Very clean and modern with good equipment, I have trained there before.
1) Australian driving licence for proof of current address.
2) Photo of Passport
3) Bank statement. I showed them 6 months of account history since mine was only 2 pages long but I think it's fine just to show 1 page with more than the 500,000 baht equivalent total.
4) Muay Thai Documents all supplied by Phuket King Muay Thai. It was 23 pages in total. I had to merge them all into 1 PDF and compress the file under the 3mb limit.