How can I increase my economy?

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You need to build structures that generate Economy like Spaceports, Economy Centers and Capital.

Other structures also gives you economy; Urban Structures, Orbital Base, Shipyards, Orbital Shipyards, Orbital Base, Metal Refineries, Robotic Factories, Nanite Factories, Android Factories and Biosphere Modification.

One important point about Spaceports. The economy it brings is accumulative. Level 1 gives you 1 econ, level 2 more 2 econ, level 3 will result in 6 econ in total and so on.

Check this table:

Level Econ boost
1 1$
2 3$
3 6$
4 10$
5 15$
6 21$
7 28$
8 36$
9 45$
10 55$

Exclamation Mark Free accounts spaceports are limited to the amount of bases the player have. So if you have 5 bases, you cannot build spaceports level 6.
Exclamation Mark Premium accounts have no limits to build it, but the accumulative economy holds when the structure level reaches the player amount of bases.
Exclamation Mark if you have 8 bases, all additional spaceport levels higher than 8 will give the same econ as level 8: 36$, when you build another base, the bonus will increase accordingly.