Are devils worth 7 blood each?

[Boron]

PDF said:
Tell me about one other good summon for such a low cost ...

I agree fully with you
3 other "good" summons of relative low cost come to my mind :
- Vampires from vampire lords
- Ghosts from wraithlords
- Demon knights / storm demons via the infernal xxx spell .

Soul contract is the easiest to get . Mictlan needs anyways construction 4 for SDR . Normally to get the second cheap +1 blood item , the bloodthorn you need to go construction 6 though to get 5B mages to forge the soul contracts .
Only vampires + storm demons are almost as useful as demons but given that you need blood 9 to get storm demons in half reasonable quantities and that you need B3D4 on your pretender to summon the first Vampire lord devils are a bit too cheap aquireable via soul contract .

Most important is also the time . In theory you could take e.g. a FoB pretender and start forging soul contracts with it once you reach 80 blood already . As you said you have your SoulContract devil economy probably running 10-30 turns before your enemies get something which is almost as strong summonwise .

You force your enemies almost to get staff(s) of storms .
Later by summoning some storm demons you can use them against the staff of storms battlemage anti-devil armies and the devils for the rest .
Basically the only thing you have to worry are enemy UBERSCs then , especially Airqueens .