@Wayne ******
There is no ongoing 90 day cost. Maybe your friend was using the agent to do his 90 day reports, which would cost additional.
Officially it is 2000 baht for the 90-day non-O visa. And then after that it's 1900 baht per year for the extension.
If you use the bank deposit method, the balance must remain at 800,000 baht for 2 months before your extension, and 3 months after. It can never drop below 400,000 the rest of the year. So that's 5 months per year at 800k and 7 months at 400k. Most people just leave it at 800k so they don't time it wrong and ruin their ability to get an extension.
But many people also start sending 65,000 baht per month to their Thai bank account from overseas during the first year. This allows them to switch to the income method for year 2 and onwards, meaning they don't have to keep any money in a Thai bank account. Each year they just have to be able to show they receive 12 months of 65,000 baht from outside of Thailand to satisfy the income requirement.