Several hundred if you are lucky. I paid around 700 USD to passport health. I have insurance but never bothered to have a primary physician. I bitterly complained about this and many people said their physician just signed it. I so hated paying so much for that medical certificate that I came in on visa exempt and did the 800K and got an O visa. But given you are still there. You might want to just keep asking.
Just a thought. 10 years out and will you still be retired in Thailand or perhaps some other country? For that reason I'm looking at a company that will insure me in a basket of countries. There are many international health insurance companies which offer this. I have not chosen one but I'm looking at April Intl Health. I would suggest to use an insurance broker as their use does not increase the price. I cannot recommend a broker as I have not bought from one yet. But insurance is complicated and I think I'd want to run it by someone who is an expert in the field and who could help me after a purchase if I needed help.
The worse part of the OA for me was the medical certificate. As I had no primary care physician I went to a couple of minor medicals who would not touch the certificate. I then went to Passport Health and they did it in about 2 weeks (maybe 3) for around 700 USD. Yes, that's true. Plenty of people said their primary care physician just signed it for no cost and I think that's valid but I just didn't have one. That was the most onerous part. Everything else was pretty easy and the process went fast with the Embassy once I had all my ducks in order. Oh, also, no one at BoA would sign anything indicating how much money I had in the bank but just printing my online statement worked. It did take a few weeks to get the no criminal record but it was inexpensive and the request could be made online. I did this in 2019. This year I took a "more money than sense" approach and just arrived on visa exempt and paid a reputable agent to get me a Non O visa and used Wise to move the money into a Thai bank account.
A lot of countries have onward ticket requirement on the books. It's hard to find out which though. But they do. And it's often the Airline taking you to the country who will initially enforce it. I went to Brazil on a one-way flight and at the Delta counter I was asked when I was returning. They wanted to make sure I had an exit. I gave them the date and they asked if it was with Delta and I said no it was with Latam .. they didn't want to see the onward ticket I'd bought. They just wanted to know. Anyhow ... here is an interesting episode on the always entertaining radio vagabond when he almost missed a flight to Colombia for this reason
I did what you want to do back in 1984. BTW, do you have work permit? I did not so my Thai partners did all the work. Personally I think it's a very, very hard business to make money at but it might help if you have a work permit and you are there day in and day out. The cow grows fatter under the farmer's eye. If you are truly diligent and a hard and smart worker then no telling what might happen. Very often your original path will veer into something else. It's my impression that people who invest in startups really invest in the people behind the startup. Were I you I'd look into how hostels make money and how much they make and how they really make it. Be aware of the ceiling. I wonder if just one hostel would do more than buying you a manager's wage. But then again it could be a platform for you to do other things. When I did it, it was really for fun. Ah .. I see you asked for people who did it in the area, I did not. I did it in Chiang Mai.
So you know what visa exempt is. William did a good job explaining how to get it in country. But you still have a lot of options. Were I you I'd start with the RTE from your country. I would also join the following FB group as many have asked your question
Wise is the most upfront about all the fees I've seen. I did a test and the 1K USD I sent from BoA ended up being worth only 969.00 USD on that day's exchange rate. And there was a fee to send but I do not recall what it was. Wise will even show you if alternative methods are cheaper. I ended up using Wise.
to check how often a flight is cancelled. Also, when I flew back I was going to choose Qatar, Emirates, or Etihad as anecdotally they seem to cancel less than others. But even then, I look at their history on flightaware. As it was I chose Etihad and all was well.