I already have a retirement visa. The application wants a departure date but if you click "returning to Thailand" for your purpose of arrival it does not provide a box for a departure date. I can only go as far as entering my arrival day and a get an error message.
I "follow" my husband's retirement visa so we only need the bt800,000 but if we each get a separate visa it is bt800,000 a piece. We have had this visa for over 4 years and have not had any problems.