how is unit-upkeep calculated?

How do I calculate supply usage? Some fantastic units like undead, trolls, magical creatures, and vinemen do not eat, and have a "Need not eat" icon. For almost all other units, supply usage is (size-1) with a minimum of 1. Size can be found by right-clicking on HP in the unit stat screen. So, an army with 10 barbarians, 1 barbarian commander, and 1 hoburg champion leading 10 heavy cavalry will use 32 supply total: 11 size 2 units = 11 supply, 1 size 1 unit = 1 supply, and 10 size 3 units = 20 supply.

This is from the newbie faq. As you can see a Size 1 unit eats as much as a Size 2 unit (Greedy Hoburgs) and scales upward.

I don't know off hand what the upkeep of Foul Spawns is, but I'm sure it's not really all that much since I've had massive armies of them and it didn't tear apart my economy.

You may be able to find their initial cost in the mod tools.

As for your Heat 1 Preference. It doesn't matter what your troop type is, if your nation has a preference of Heat 1 then all your provinces behave the same even though they may be made up of different species and nations.

Also if you are in your domain you shouldn't see too much of an impact as far as supply and if your provinces are in your domain a gold issue.

In order to counteract moving out of your domain and losing supplies, you might want to use some of your nature gems to make a few winebags or cauldrons. Or alternatively design a pretender with a quick and easy castle (Like a Wizard's Tower) in order to keep your supply where you need it to be.