... it'd still be horribly imbalance to have planet creation tech automatically create planets that are barren. :/ Making the barren pick even more expensive to compensate is a poor solution.
Asteriod belts could contain lots of rock, and some ice. Creating ocean, swamp or terran worlds from icy asteriods seems reasonable (to me ... and I'm quite reasonable )
. Rocky asteriod belts could create barren, radiated or inferno. Then less icy asteroids could create desert or tundra. Toxic is a problem though
My idea is that the composition of the asteriod belt could determine what kinds of planets could be built from them - along with size determining size (obviously). Some leeway should be allowed, since you don't need to use all the ingredients. I think controlling the temperature - i.e. choosing inferno over barren is fine since if you can build a planet out of asteriods I'm sure you can move it a bit closer or further from the star! Maybe 'potential ocean' asteriods could make smaller terran or swamp worlds.
As an aside;
The different types of planets are a bit random. Desert/tundra is a difference in temperature, but swamp/ocean/terran could just be a difference in layout - maybe ocean has more water, maybe it's just more shallow. Toxic and swamp have different atmospheres to other types. Radiated could be anything, with lots of radiation (like an extreme solid core special).