I added a post, to this and it's not showing up in new posts?
600 stone, per level, is excessive. Plus there isn't enough storage, in stone age, without adding 2 wool, to whichever town, he turns up in.
Stone age altar, will get you to level 15, no point in spending resources, when the altar will do the job, so the first time you will want to use the mentor, is level 16, cost 9k stone. Level 17 9,600 stone, etc.
Max stone storage, without using blueprints or wool, is 3,800.