You can increase your bases economy by building the following structures:
Crystal Processor
Economy Centers
Spaceports
Empire Center
Also those structures will give you more economy:
Habitat Zone
Orbital Base
Shipyards
Orbital Shipyards
Metal Refineries, Robotic, Nanobots and Automated Factories
Biosphere Modification
Tele-Transport Pod
The most impressive structure to economy is the Spaceports. It gives only 1 economy to level 1, but each next level will increase to match its level, like 2 to level 2, 3 to level 3, etc.
So Spaceports level 5 will give you 1+2+3+4+5=15 econ.
Free accounts spaceports are limited to the amount of Bases, Premium accounts aren't, but the increasing economy it gives stop at the number of bases, so if you have 7 bases and each base have Spaceports level 10, after level 7, all the other levels will give just 7, not 8, 9 and 10. Upon you build your 8th base, all bases will get 8 to each next level above 8.
Remember to proper defend your base based on its total economy, as bigger economy will give bigger pillage to the enemy. O-P-Rings are a must if you have economy 100 or higher. Also add some small and middle power batteries plus Deflection Shields, so all batteries will have shielding making your base less profitable.