This achievement is not cumulative. You must have a total of $100,000 at a single time. You can make money by selling items like jewels to the Librarian. You can either grind for items to sell or use the following glitch to get a huge number of gems to sell at once.
1. Try to have only one of any valuable gem in your inventory and equip the Sword familiar.
2. Stand in front of the Master Librarian and use the Sword Brothers spell (see Warlock).
3. Quickly approach him to bring up selling menu.
4. Go to the jewel selling menu and press . Equip the jewel you want to sell.
5. Press again to close the menu and sell the jewel, then exit the shop. You now have 255 of that game to sell and makes a big profit.
Use this approach as many times as you need to to get the $100,000.