Are devils worth 7 blood each?

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Even if there are ways to deal with Demons, the fact is that they are bloodily (!) cheap when you got them through Contracts. I do think- and other experienced players too- that Contracts are somewhat overpowered. The contract takes some time to pay off for sure, but with a Blood nation you can churn a very high supply of slaves and transform them into Devil-factories without even having to research Blood. Best of all you have to use 1 mage-turn to make the contract but no time is lost summoning the devils, that come automatically after that.
It's not a rush strategy, but an income of 60 slaves/turn is possible relatively early, allowing mass-production of though devils. Tell me about one other good summon for such a low cost ...