I currently have a game with several planets very close to stars (including mercury), where the temperature is around 360 C, and the colony cost is 2.7. Isn't that a little low? The colony cost for planets under the minimum temperature goes up much faster, mars -70 with 2.7, that's only 60 C from the species minimum. Europa at -164 has 6.41 but is still closer in temperature than mercury.