Using only DPS and the cost, what's the formula to determine if it's better to place multiple of the same tower or purchase an upgrade?

Example data:

level 1 - total_cost: 2000, dps: 6.66

level 2 - total_cost: 2250, dps: 6.66

level 3 - total_cost: 2775, dps: 13.33

level 4 - total_cost: 4775, dps: 16.67

Level 3 + Level 0 is more efficient than Level 4.