Blood Slave Hunting Data for Dominions 2.  Seems to be a little different from Dominions 1, though not much.  My analysis is not as complete as Sunray's.

I suggest you read this a constant-width-character font, like Courier New, so that the chart is aligned.

Setting:  Mictlan, neutral dominion except for +3 luck and +3 growth.  I captured 6 provinces, of the following populations:

3.5k, 4.5k, 6k. 8k, 10k, and 15k, for a total of 47k.  I set taxes at 0 and patrolled each with 65 units (mictlani slaves).  In addition to the tribal chief with 65 soldiers, each province had a lab, 6 scouts, and 2 mictlan priests.  

Running from turn 39 to 59 (most times), I set either all the scouts, all the priests, or all the priests with SDR's to blood hunting.

Data:
Unit Type          Units/Province   Harvest (turns)   Slave/Unit-turn
.
(Six provinces)
36*scout           6                790  (20)         1.10  
12*priest          2                487  (20)         2.03
12*SDR-priest      2                1029 (20)         4.29
12*DualSDR-priest  2                1068 (20)         4.45
.
(One 8k province)
6*scout            6                238  (40)         0.99 
12*scout           12               238  (20)         0.99 
18*scout           18               341  (20)         0.95  
6*SDR-priest       6                522  (20)         4.35
.
(One 25k province)
4*L10-blood+SDR    4                510  (10)         12.75
.
Maximum harvest in a single province:
~110/turn average, using 12*(L7 to L12) blood mages and 36 scouts.


In summary:

Scouts are ideal.  Never use blood mages without SDRs.  Blood mages get more bloodslaves per unit of unrest, in general, so you can get more slaves from a given province.  The upper cap seems to be about 110 slaves per province, no matter how big the province is, but below about 4000 people the harvest starts to drop.  In other words, I was getting ~100 slaves/turn from a province that started at 9k, and dropped to 600 people, but the blood harvest did not suffer until it hit about 4000 people.  I cannot make any declarations about the cheapest way to get blood, since that depends on many factors.  

However, these are a couple estimates:

The steady-state cost of bloodslaving using 8 scouts to hunt and one ferry, in a 5k province, setting tax to 0% and not patrolling, including only upkeep and lost income (not population loss), would be:

8 slaves/turn at 4.6 gold per slave, setup cost 180g.

3 SDR-equipped Mictlan priests would yield, instead:

12 slaves/turn at 2.9 gold per slave, setup cost 260g + 15 slaves + L4 Construction.

The total cost over 30 turns of bloodslaving using 12 scouts to hunt, one ferry, one commander patrolling with 35 archers, in an 8k province, setting tax to 0%, including only upkeep and lost income (not population loss), would be:

12 slaves/turn at 8.1 gold per slave, setup cost 570g.


Generally, expect 1 blood slave per unit per turn, plus 1 per level of blood.  That means Armor of Souls takes 40 turns to pay off in blood hunting.  And from my testing, SDRs have no effect (or maybe a negative effect) on non-bloodmages.

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