And then you can add the costs of the business and work permit, as well as the required employees.
Having run a business established under the Amity treaty, my expenses ran about 5,000 baht per month for accounting services, roughly 50,000 baht per year for the work permit and extension renewals, and then the costs of having my employees (salaries and taxes). The costs are not trivial, and can be a bit painful if not supported by actual income from the business. On the other hand, if the money is flowing, then you can easily pay for these headaches to disappear.
Oh, and then there is the cost of setting up the company. As mentioned, 2M baht for proper capitalization, and the costs of establishment, which ran roughly 180,000 baht in my case (it can be done cheaper).