Colony Revenue

[Quicksilver]

Would I be correct in assuming that the minus sign before amount indicates a negative yearly return?
ie: -66 for first fifty years -16 after.
So that even with four mines the real return will only be +4 per annum once it graduates out of colony status?
The only worlds that I have noticed with positive earning numbers are type 5 and 6. Why would the most uninhabitable planets be the most valuable?
If the above assumptions are correct then the optimum potential colony would be ones with the lowest post colony upkeep,since positve numbers are very rare.
Some clarification would be appreciated.

[Kahn]

I believe the reasoning for this is the developers wanted to make the resources harder to get to (High resources, low habitability), basing their logic that anyplace that would be desirable to live would not be the best place to find resources. A good allegory to this is oil, outside of LA and the black sea, few oil rich fields exist under major population centers. Most are located under deserts, contenetal shelfs, or steppe, not really locations that one would picture as great places to build a city.