[Baalz]

Well, the real factor that starts coming in for sub 3k provinces is the value of your mage's time rather than upkeep. You'll average a little over 16 blood slaves per turn for a 5k province with 3 Mictlan Priests with rods. You'll get half that for a 2.5k province, and it's costing you 3 mages that could be researching (if nothing else). Really, the cash generated by sub 3k provinces is negligible anyway, its a factor of if you have something better for your mages to do (you almost always can use more research).

You will be getting much more blood slaves from a single blood hunting province than you can conceivably hope to get from all the magic site income. There are very few magic sites which produce blood slaves, and none produce very much. They can produce quite a savings on the spending though.