Tap Paradise CoveMore guides, cheats and FAQS
Upgrading your merchants costs pearls. You have to upgrade the merchant 5 times to get to the next tier. Once you reach this next tier, you see the increase in income. With each increase in tier the number of pearls required to upgrade again goes up. The maximum level your merchants is tier 6. For the 8 hour merchants, it appears they follow similar income/pearl costs.
For example: Sugar Mill Tier 4 gives 7664 every 8 hours. It costs 141 Pearls per upgrade. Stone Quarry Tier 2 gives 7664 every 8 hours and costs 141 Pearls per upgrade. Copper Mine Tier 1 gives 7664 every 8 hours and costs 141 Pearls per upgrade. After another upgrade, they all go to an income of 9960 every 8 hours which is the same as what the Fishing Pond Starts out at.