To be honest, I hate this idea.
Sure Ulthuan probably can afford to pay. But charging them for a service they are doing for the entire planet, even if they get some side benefits out of it, is not something I will support.
Yes...Ulthuan is using the magic for things such as... Keeping the continent with countless civilians and the infrastructure that allows the vortex to exist in the first place. Along with spells and weapons fighting Chaos and keeping the Seas free of Druuchi and Norscan pirates.
This seems like attempting to charge the company that takes your trash for the privilege because they recycle some of it and sell it off, before using said funds to keep the whole operation running.
I feel like I'm kind of of two minds about the idea.
On the one hand, it strikes me that Ulthuan is basically in the exact same position as the Karaz Ankor, if for different reasons. They're both maintaining a network to keep themselves alive, where one will turn to stone if it gets too low for too long and the other could sink into the sea. Yeah, both have other uses for magic and probably various great works they can employ it towards, but at least with the Karaz Ankor the gap between "last of the great works shuts down" and "slow, civilization-ending power failure" was... not large. Coming in in the middle of that and using the Empire's position "upstream" in the network to mess with things or coerce a deal definitely doesn't sit well with me.
On the other hand, if a surplus of energy is put into the system, eventually, there will be negotiations over what to do with that windfall of magical power, and even just thinking about things like an overflow link to divert excess from, say, the Kislev network in Praag to Karak Vlag, I can see a lot of negotiation occur over exactly when and how bleedover is handled. Or if we find that something is up with the Tilean network and reroute the Border Princes towards Barak Varr, for that matter. Is this a temporary situation? What happens if and when it gets fixed? etc. Even in building out the system to benefit literally everyone there's a lot of favors that can be traded to negotiate exactly when and how that happens.
If the hairs start being split fine enough, or the system has any significant flexibility in where energy goes, one of those favors will almost certainly be money.
So my overall feeling is we probably need two aspects to any negotiation with Ulthuan: One for bad times and triage (read: the status quo or worse) to ensure that harm is mitigated, even if there's not enough. After that, we can have another, looser, part of the negotiations for what to do in good times. As long as it's not trying to force their hand, I feel like it's something that's just going to inevitably happen if we involve them. It's struck me as things get better and polities advance, that flow of energy may graduate from being "the thing keeping us all alive" to being regional electrical grids.
Sure, we're a long,
long way where the Kislev and Ostermark can suck down a Storm of Magic and have one of their chief concerns be what to do with the excess, but payment for a power surge is what might happen if things even begin to approach that point.
Of course, for all that to work out happily, we probably need both knowledge of when and where things would need triage --- such as learning about the Karaz Ankor's near-apocalypse failure and Ulthuan's "desperately avoiding sinking into the sea" situation IC --- and some examples of work that can be done to drastically improve the status quo. The latter of which would probably generally be areas like the Badlands, where we know things don't have networks, or finding novel areas that don't terminate in Ulthuan. (Which is part of why I'm so keen on the Tilean mapping to see if Skavenblight is sucking up regional leylines to make warpstone. It's entirely possible there's a flow eastwards from Tobaro or Sartosa, but If a flow to Skavenblight exists to be cut loose, large or small, then the prospect of doing so becomes a bargaining chip. Still, even that's just illustrative, really. Any flaw found in a mapping action can easily become a bargaining chip once we're in a position to fix it. Who's chip depends on who fixes it.)
But I digress. The core point is that this is foreign policy. Yeah, some of the prospects are creepy and some options are unsettling than others, (and, to be fair, probably less stable? Coercive force has a long history of being unexpectedly fragile, once challenged,) but such is foreign policy. Negotiation is going to happen. Ideally, said negotiation ends up with everyone invested to maintaining the new status quo, but for that to happen, everyone has to have something worth maintaining.
