The cheapest is doing it yourself. Cost you less than 3000 baht for everything. But if it's your first time doing it on your own, buckle in. You're gonna get frustrated 🤣🤣. If you have a Thai girlfriend or friend, or friend who speaks Thai. Bring them with you to translate. Saves hoirs.
I assumed he was talking about the extension. If I remember correctly, I wasn't required to provide proof of funds at all when I applied for the non-o. Been a couple years but I had already opened a Thai bank account before applying for the non-o from my country.
To be honest, I almost did on my first retirement extension. It was getting pretty frustrating. Now that I'm familiar with it and remind myself I'm not busy 🤣🤣🤣 so I don't need to hurry, it's pretty easy.