Where does it say again that the city of Sigil has sucker 17th level casters that will happilly answer your summons?
no gp limit applies when buying or selling goods in the City of Doors, and NPCs of any class and level combination can be found there.
You know what that means right?
There's 175 base classes in D&D, not counting prcs.
The city of Sigil has
at least one 17th level NPC for each possible class combination.
This means the City of Sigil has 175^17 17th level NPCs, aka 1,354 at the order of 38.
The City of Doors has hundreds of millions of millions of millions of millions of millions of millions of 17th level NPCs.And even more 18th level NPCs. And yet more 19th level NPCs. And let's not mention 200+ level NPCs.
Screw the Lady of Pain, I don't want to risk messing with that overzerg rush of high level NPCs!