The EconScore is *solely* based on your SumOfTotalIncome - which is a value that *cannot* be in the negative. The expenses you have are not taken into account.

However, in this example you state

clearly the expenses have been calculated in, and this is misleading. The only conclusion that could be drawn from this is that the Altarians were much more willing to spend money as you, or, were bunkrupt at some point, like when you loose some planets and find yourself in the situation to lack the funds to upkeep your still existing Military. Still, this is very vague now from me spoken, generally most of these end-screen statistics can be savely ignored.

What is much more telling is the "Stats"-section in the 'Foreign Policy'-section. Of course it won't give you scores, but will tell you hard numbers to the first 4 'Timeline'-curves, onto which the EndScore is based.

Another thing is that *most* of the Econ-score is derived from taxes. Given a high difficulty-level the AI will have a fair economic-bonus which makes him eligible for a higher Econ-score even if his taxes are way lower than yours.

Basically that's one of the reasons why it is so hard to get an Econ-treaty in suicidal.

Correct.  All of them, actually.

Techs researched, techs stolen, longest tech, most productive research colony: irrelevant.
Ships defeated, ships lost, most powerful ship, soldiers killed in battle: irrelevant.
Net income, most profitable month, leases owed, average tax rate: irrelevant.
Most populous planet, happiest planet, average approval, most influential planet: irrelevant.

Research: Gross research (NOT research spending, due to bonus production being half free).
Military: Military might.  Determined via Attack + Defense + (HP / 4), per ship, times a known modifier, depreciates based on number of ships (actually number of ships plus one).
Economy: Gross income.
Social: Total civ population.