3 stages was with books.
1. Then u could farm from mobs, was cheapest way, dosent matter if they pk u in top spots u had plenty time farm in lover spots for make skills on main char.
2. From taro , that Leflyll mentioned for 2kk most books price was,( i bought even for 1.7kk and made skill. ) So around 500b per skill which is perfect price.
3. And after then im jump insance price ( from altar zone ofc) books went to 1b each. P.s we all know that altar been nerfed by adena drop, so try now farm 1Tera adena for one skill :)) If u cant solo kill mobs in TOI - nothing left.