This guide focuses primarily on the cost per income ratio and ignores build times.

Credit factories are taken from the legendary Moo's Defence Agency mod. Here is a short description from the mod page: "Large and expensive building that generates credits at a very high rate. Can be upgraded up to T3 to increase income. Explodes violently if destroyed, its power increasing with every upgrade. Defend it at all costs!"
Their specs are as follows:

Credit Factory T1

• Total cost: 25,000 $
  
• Total income: 35 $/tick

Credit Factory T2

• Total cost: 47,500 $
  
• Total income: 60 $/tick

Credit Factory T3

• Total cost: 70,000 $
  
• Total income: 85 $/tick

The Google Sheets document that has been used to draw the graph can be seen here[docs.google.com].
Feel free to contact me if you spot an error.

At the beginning of a game, players do not have a lot of money and are therefore restricted in what they can invest for the economy. Let's say they are not willing to spend more than 10,000$.

• The first fact emerging from the graph below is that extractors are the clear winners, because they are very cost effective. Especially at the beginning, where most resource pools are still free, it is smarter to build more T1 extractors instead of fewer T2 or T3 extractors.
  
  
• Another interesting point is that T1 fabricators are not bad at all! For example, if you want to increase your current income by 10, it is more convenient to build multiple T1 fabricators instead of fewer T2 or T3 fabricators. It is therefore recommended to fill that empty space inside your base as soon as extractors have been brought to their maximum.

At a later stage of the game, where players have enough resources to invest thousands for their economy, it is crucial to know which economic strategy is more efficient.

• Extractors are still the best choice, but we can assume that resource pools have been occupied and are not available any longer.
  
  
• Surprisingly, T2 fabricators overtook T1 and T3 fabricators! This means that if you have still enough space for new fabricators, it is more efficient to add T2 fabricators instead of upgrading your current fabricators to T3.
  
  
• Credit factories are not as good as you might have thought! It turns out that T2 and T3 fabricators are still the better choice. To be honest, credit factories are also quite dangerous because of their death-explosion. So whether T1 credit factories are preferred over fabricators heavily depends on the map and on the combat strategy that you plan. Maybe you don't have enough time to build dozens of fabricators and might be better off with having a credit factory in some secure corner of the map.