The win of an open source "game making" tool is surely not the low cost, but having access to the source and enabling modifications and porting to different platforms.
The core of RPG Maker is closed source, but a surprising amount of it is written in user accessible Ruby.
See mkxp (https://github.com/Ancurio/mkxp), an open-source replacement for the RPGMaker engine that was used by "To The Moon" for Linux/Mac support.
As I understand it, this is not true of RPG Maker 2000/2003 (which is what EasyRPG is attempting to target). Ruby support was introduced starting in RPG Maker XP, which was released after 2003.