Clear levels 1 to 10 to unlock levels 11 to 100.

This is an old trick used back in Wizardry (using character pools) and requires a lot of time but the pay-off is worth it when you begin the game.

Create one of the characters you want to play as through the whole game. Use the other slots to create characters until you cannot do so anymore. View each of the other characters, de-equip and sell all of their stuff, transfer all their wealth to the party, and delete the now useless character.

Create new characters and jeep doing this until you have the amount of money you want.