I've looked into negative trade values and its relatively easy. However, what isn't easy is how to deal with all the trade modifiers. As it currently stands, the modifiers will all make a negative trade income worse which precludes the idea that a value may start negative and work its way positive through modifiers. What behavior is desired?
My desired behaviour, take for example
Gives a Starbase a trade value of 1.25 credits per tick (however it is calculated)
What I would like is if that value said "-1.25" for example, that it would take away 1.25 credits per tick, similar to how an undeveloped planet drains credits with it's negative tax when undeveloped.
Since each level of the "BaseTradeIncomeRate" upgrade property seems to overwrite the previous, the number could be reduced each level after paying the upgrade cost as an efficiency boost reducing income.
Basically all I'd need, is for a negative number in the "BaseTradeIncomeRate" upgradeproperty to actually subtract it's value from credits (and preferably if possible show up as a negative source in the credit summary.
If the research trade modifiers affect this negatively, well I am ok with that, as boosting your trade would then increase the penalty rates for upkeep thus balancing it >=P