Everything which produces economy will eventally generate the credits that it cost to build.
Structures with payback[]
All of the structures that provide economic increases eventually pay for themselves. Crystal Mines (+crystal resource), Spaceports (+2) and Economy Centers (+3) are the big three.
Nanite Factories (+2), Android Factories (+2), and Orbital Shipyards (+2) are worth mentioning, but usually they are constructed to boost production. They will, however, pay their own creation costs. So will Metal Refineries (+1) and Robotic Factories (+1) and it would appear to take twice as long for them. But remember they cost much less to construct, so that is not really true.
Spaceports (+2) give the additional benefit of allowing Trade, which also boosts your income. Remember that trade routes cost credits too, so they also have a payback time.
Ships with payback[]
There is only one ship which makes money directly and that is the Recycler. It pays for itself in 3 hours, if it has access to debris.
There is, however, the possibility that all combat vessels will generate income by the occupation and pillage of enemy bases. The amount they are responsible for generatiing, and how it relates to the overall cost of shipbuilding, is a very difficult calculation.
Calculating Payback Time[]
Economy Centers cost 80 credits for the first level. They produce an additional 3 credits per hour once completed. So the payback time for this level is 26.7 hours. You should also remember that you will get no income from that level until it is completed, so it will be well over a day before that construction pays for itself and your Economy Centers start to make you real money.
For higher levels it is longer. Level 3 Economy Centers cost 180 credits.
That level gets you an additional 3 credits per hour so payback is 60 hours + construction time. You might think that you are getting 9 credits per hour, but remember that those earlier levels had to pay for themselves AND THEY MIGHT NOT BE DONE!