Is it the case that civ infrastructure on roids is every bit as relevant as on planets?
Yes, because it eliminates the credit income drain
The thing is Is it still profitable to just build civ infrastructure to the 2nd last level on planets? I think thats the stage where cost goes to 0.0 (Will still make some kind of money on that planet or does that just mean it will in total just break even?)
The 0.0 means the planet isn't losing any due to underdevelopment. But since the planet's population will grow, it will still make some money. The higher your infrastructure, the more population the planet can support, and thus the higher the credit income out of it.