Some methods that I know:
1. Start by earning at least 500m for a High Wizard(one time payment for each character), then do and finish the Endless Tower Quest weekly and obtain any stuff you can sell (E.g. Valkyrie Stuff, Entweihen Crothen Card, Fallen Bishop Card,etc)
2. The Buy and Sell Method (Buy for a cheap price then sell for a higher price)
3.Make and Sell(Hardworking Money Making) = Make some stuff (Custom Items, Castle Drops from WoE) then sell them for a reasonable price