unit upkeep cost

Yes, if you have negative gildar, eventually your armies will begin to desert.

Does it affect city unrest, too?

late in the game, units can be pretty expensive. i've played games with relatively large amounts of units where my upkeep exceeded 40 gold per turn. you need a few big towns (and preferably also a few trade agreements) to generate enough cash for such a big army. 

as you add more traits, armor, mounts and extra items (like the recently introduced rations & med kits), unit wages can get pretty high (i've used high end designs that cost up to 3 gildar for a single full 6 man company) and losing the ability to buy items for champions or rush buy crucial build queue items in cities is already a big disadvantage. 

double post

hmm i see whats best way to make money than

primary source of money are cities specialized as towns. they get higher base income with each level up and unlock the market and tax office buildings which further increase income. and they have access to a line of buildings (well>inn>festival) that increases money income by a percentage.

so towns are your safest bet. secondary sources of money are the "propaganda" city enchantment of cities with several essences (+1 g per essence), gold mines that can sometimes be found as bonus resources (similar to shards, iron mines, crystal crags etc.) and of course also the loot you get from killing monsters and clearing lairs (this includes selling unneeded items at the shop of a city).

and you can make quite a bit of money though diplomacy. each trade agreement you sign gives a 10% boost to your income, so try to make some friends and sign those treaties (preferably AI nations of your side - kingdoms if you're a kingdom, empires if you're an empire - the "enemy" side tends to get unfriendly/hostile later on)

and you can also sell unneeded resources (excess iron for example) or tech knowledge to AI factions for gildar.

i think that's about it.

Other sources of income/conservation of money:

• Changing your tax rate via the top-right corner. Just be aware doing so increases unrest which causes less research (and production?) per turn
• Quests that give money as a reward or option
• Choosing the "Produce Wealth" option in a city's build queue
• Alchemy spell (I haven't used it recently, but it was around for a while where 100 mana = 100 gold)
• Disbanding un-needed units can conserve money spent per turn

I think those were the only ones missing from Azunai_'s excellent list.