Binding already cost thousands of nuyen per attempt, and it didn't stop anyone. Multiple Spirits without the whole 'I'm rich as fuck because I got nothing to spend my cash on, so can afford to spend downtime binding a massive Spirit army and just throw an army down whenever I want' isn't a balance problem.
As for a Force 12: Give me 5 karma for an Expertise and some karma for +4 Summoning Foci, for 19 dice, and with just a mere 2 Edge expense I can hit 39.71% chance to score at least 1 service. As long as the character is solid enough to survive the drain, which let's say 4 Initiations will bring to ~25 dice for an Elven Shaman... Let me check how well they can handle at that point... 1.15 drain taken on average then, without applying Edge.
Given how you could spend 50*12=600 nuyen per attempt to get at least 1 Edge, I'm going to assume we always force 1 reroll since the GM won't let us keep Edge once we start resting... Odds of the opponent rolling 0 hits are incredibly small so we can just always apply -1+1d{0,0,1}... That makes it 0.88 drain on average, with about 1/7 chance of 3+ drain taken. So given 8 hours of preptime, this build can summon, rest, summon, rest, repeat until the roll difference is in its favour and then throw down 3 Edge (1 from the Reagents and 2 from natural Edge) to make the roll even better. There's 24% chance to score at least 1 net hit in the 19 vs 24 test, so let's assume we do 5 attempts for 3k Nuyen. We then have 1+ service, and on average score 2 more thanks to forcing the Spirit to reroll 3 of their hits.
So I think an F12 Spirit is still quite doable with minmaxing: Get to 25 drain dice with 5+8 attributes +4+4 Increase Attribute Focused Concentration +4 Centering, get to 19 Summoning dice with Focus and Expertise, 63 karma.
I think as Houserule I'd make oversummoning 'Due to forcing your inferior magic upon a creature from the Metaplanes, you generate (Spirit Force - Your Magic) in Edge against you that is used on rerolling your hits'. That makes oversummoning very very hard very quickly, even if it won't impact the drain, just your odds.
https://anydice.com/program/18317 for the math I used here.
Edit: Note that I didn't even take Mentor Spirit (though the question is, what does edge boost discount mean, is it 'rerolls won't be affected since 1-1=0 but minimum is 1', or 'rerolling X dice will cost X-1 edge, minimum cost 1') or Spirit Affinity into account in the math here, which makes it even easier. If anyone wants me to math out Chargen 19 Drain Dice (no Elf) and Chargen 14 Summoning Dice with Mentor Spirit, Affinity and Reagents on an F10 instead, feel free to ask if you consider it useful for the debate.
