$19 per month for the monthly subscription of using "The Editor".
Create my own editor for your "open source front end".
No, thank you. That's what I think .
Further comment: It seems to be more targeted to designers, not programmers. It looks kind of limited to me, because the game can only support only the things "editor" and "realm" does support. Getting into it and hacking additional stuff requires a lot of knowledge, plus getting familiar with your API.