Details please from those in the know.
How much do they cost and how long are they good for?
At the risk of repeating repeating myself: does it expire and when?

I think I am asking two questions: how long is it go for leaving and returning, once you buy it and once you leave?
I think I only need a single but details on multiples are welcome.

There are 2 types of entry visa's.

Multiple entry and single entry.

Regardless of which one you get it will expire when your retirement visa or other visa does.

single entry is 1000 baht

multiply is 3800 baht.

There is no time frame on how long your gone as long as your back before your retirement visa or other type of visa expires

There is no time frame on how long your gone as long as your back before your retirement visa or other type of visa expires

The re-entry permit will have an expiration date that is equal to the date that your permission to stay (ADMITTED UNTIL stamp) ends.

Just got mine renewed today. 3800 for a multi entry