How does upkeep work?

[DBear]

Disclaimer: I won't be getting Civ4 for a while, since I KNOW my present computer is incapable of playing it. I just happened to not see this answered anywhere.

Various civics have either no, low, medium, or high upkeep. What is the difference? And what happens when you combine civics with different upkeeps? Are they averaged? Highest one dominates?

 

[mrjepson]

Each one seems to add its own portion. All high is much more than one high and all low.