First, unlock and activate as many stud multipliers as possible. Then, travel to Bree to find two purple studs (worth 10,000 each at x1 multiplier) in the center square of town, and collect them. After collecting the purple studs, enter the nearby interior shop, and then go back outside to make the purple studs respawn. Repeat this process as many times as desired. If you have unlocked and activated all five stud multipliers, you will earn 38,400,000 studs for each purple stud collected.