It's still not making sense now you're going on about 3,800 That's the price of a multi entry not a single entry. An entry permit is 1,000 bht or 3,800 for unlimited re-entries.
There is no need to show a one-way ticket leaving the country or any other tickets All you need is to have a re-entry permit in your passport and that's only if you have a valid visa and extension normally on a non-O.
You won't even be granted a reentry permit if you don't have the appropriate Visa.