I have been trying to find an equivalence between Blood slaves and gold. This is what I've considered:
1.- 3 Blood hunters per province, level 1 and with dousing rod. My tests show that they get 3,645 Blood slaves each aprox per turn.
2.- Consider a mage cost of 90 gold, sacred, that will live for 15 turns (most will live more but just choose that number from now on). The SDR has to be paid too. That makes for about 33G a turn for 3 mages.
3.- Consider a province near 5k that gives around 50-60k gold a turn put to tax 0%.
Giving all this a blood slave costs from 7-9g depending on the values used. But the important lesson learned here is that biggest part of price came from taxation loss.
I now compared that cost with unit cost (let's forget about free upkeep for now), so for example at 8g cost:
Sucubus costs 528g, storm demons 56g, ice devils 704g, dark vines 48g...
So first impression is that commanders are really expensive(compared to their recruitable cousins, 704g is about 40% more than a Niefel Jarl) and units in general cheap. That is of course balanced with the free upkeep if the troops survive that is