No... Because if there was no resource cost attached to it you would be able to pay just 20 resources to convert them to full TN Construction Factories worth 120 resources.
Ah!
So in order to make it fair, you could make it, that conversion basically costs the same as a full construction factory.
My goal wasn't really fairness though, my goal was to find a way to make some asteroids and planetoids useful for more than just slow accumulation of population and (low gravity) infrastructure...
As for NPRs building them... they won't?
I enabled building CI's in my latest game, with a rule that every colony with more than 10m *must* have at least one per 1m pop (currently to a max of 2 per million), can't build in excess beyond pop growth, and can only convert into other things in case, say, you're under attack. And must be rebuilt asap once the threat is over - though I'm not there yet to see how that would work out.
But I put a cost of only 40 BP/wealth, considering I want them built everywhere without minerals. I first tried a cost of 200 but then they never managed to follow pop growth or were too much a detriment for normal development. Since CIs are way less than 50% as productive as TN facilities, this cost seems adequate as long as it's not abused, giving them a 2.5% growth rate for basic tech (if all they do is build new ones). It made things interesting with every little colony having something to do. But I'm playing with very low tech: if you play higher tech and want CIs to be interesting, maybe you should either increase their cost or reduce their factory value (default is 10% of a TN factory).
I even gave them a very small maintenance stat so they could help maintain a small scout or fighter parked over them. It makes sense that even a non-military colony could do that on a small scale, and it enriches the story to use them as pit-stops for my patrol ships. Though I'm always turning off MSP production (and fuel as well) since most places will never have use for these (or don't have the minerals) and I'd rather fire off the minerals somewhere else for better use. So they don't make proper fleet bases.
And no, I didn't observe NPRs trying to build them. Probably because they're not programmed to do so. Though I've only seen a single NPR so far, and the war was too brief and bloody to even consider converting CIs to anything else.