Here is the unit upkeep formula I have found through testing, have yet checked if this is true for naval, air units or missile (the last 2 have had a bug with unit upkeep, but that should have been solved with latest patch). Anyway the upkeep formula for armies is very simple and I would assume the case is the same for navies and air units as well.

For armies, each unit added into the army beyond the first unit reduce the total army upkeep by 5% and this work adjective, an army with 3 units will have a 10% reduction, 4 units and it become 15%. The largest possible army is 9 units with romans which will have a 40% upkeep reduction. Also the roman reduce all army upkeep by 30%, this is multiplied and counted after the reduction from number of units have been done. Thus the total army upkeep will be:

Army upkeep = (Base army upkeep) * (100% - 5%*number of units above first) * (100% - 30% * if romans)

Base army upkeep = sum of the upkeep of all units in the army.

number of units above first = is the same thing as total amount of units in the army minus one, aka the first unit in the army don't contribute towards the upkeep reduction, thus single unit armies will pay 100% of their upkeep cost.

If romans = romans get an 30% upkeep reduction as part of their legacy trait.

The Roman legacy trait was changed?  

Bummer, looks like it's now 

• +1 Unit slot available for each Army
• +50% experience gain on Unit

Also, from what I'm seeing, (another huge bummer) is that grouping units does not reduce upkeep anymore.    Why on Earth would they change this...  Upkeep is killing me. 

I'm new to the game, but I've read a ton of posts/articles, and everything is from a year ago, and it seems like so much has changed about the game in a year.