Basically it's the school fee plus a variable changeover fee from official groups inside Thailand that assist with this type of thing whether you are new or doing another year. Because it's exactly the same process less the travel and they are doing the same for everyone.
as always "easy", is not cheap. But considering other options are 50k to 70k, for longer term stays and no worries for 5 or 10 years many people are going this way. Nothing secret here.