[apoger]

>the soul contracts could become very cost-effective, particularly the 80-slave one I found.

They are cost effective over time even at full cost.


>Those demons or similar generally cost about 5 blood each to summon,

They cost 7.


>The difficulty is if the contract goes to lab or straight to the creator when built.

It goes to the lab.
It's a cursed item, once held you cannot transfer it.


At full cost it takes 12 turns for the contract to pay back it's cost. After that it's effectively an income of 7 blood slaves per turn (in the form of a Devil). This is one of the classic ways to make blood magic pay off economically.