Wow, people are really peeved that Bretonnia didn't join for free when we asked.
I get that people want some Bretonnia Shinnies, but ultimatly, I think getting them to buy in as soon as possible is much more valuable to the Waystone Project than waiting to get a finished product and blackmailling them to give us stuff for information that everybody else gets for free (sure they helped, but we also paid to bring them in so it balances out in my opinion.
Sure, Bretonnia fought the Empire in the past, but so did the Eonir and Kislev. The Waystone Project should be above those petty concerns of rulers, an idealistic project to repair worldwide infrastructure and to adress global problems collectively. Not some petty shit that ends up delaying implementation in Bretonnia just so Mathilde can wring out some books for her library or something.
EDITED : Because I was kind of a jerk in conveying my point.
I'm not annoyed at them at all. I'm simply approaching this rationally: Bretonian shinies are worth more than the help they can give to a half finished project where much of the base research has already occured.
Second,
we are a grey wizard. Hoping for the best is all well and good, but this naive idea that everyone will band together versus outside threats has been shown to be wrong again and again. We cannot trust in people acting good or out of common best interest, as we all know the tragic story of the common green. What did Bretonia do during the Great War vs Chaos? A war that if lost, they were going to be overrun? Nearly nothing from what I can find on the wiki. Kislev knows this best of all, which is why they offered support initially
only if the project was to be based in Kislev because they knew they were going to be afterthoughts.
The only solution is not to trust in mankind suddenly caring about the common good (because history in universe and IRL has long shown that they don't). No, we use a project to force others to act in the common good in order to get access to the waystones.
An actual nonaggression pact from Bretonia for the Empire. Maybe a promise of aid in the next chaos war for Kislev, but promises are fleeting, so they might want something else more immediate. Likely support for Karak Hirn for the Dwarves, or perhaps an Errantry war to retake a Karak. And something for the Eonir (though that might be the Empire, Kislev, and Dwarves owing the Eonir some favors for the Eonir's share of Bretonia's payment).
Third, requiring payment causes buy in. Things provided to you for free aren't considered as valuable, even if they are dead useful. Does this matter a ton when it comes to waystones though? Surely everyone knows to want one? Well, as we can see from the Empire Noble that wanted to carve his waystone into a statue of himself, free stuff isn't valued, even if it is valuable. To him, it was just a free rock. If, in contrast, that rock is an expensive rock that you had to pay for (i.e. a luxury good), then it becomes valuable to the Noble, who can brag about it, and to destroy it would be just as dumb as melting down the gold silverware, but also illegal. Take a page out of the Chaos Dwarves book: export Waystones as luxury goods and everyone wants them. Is it dumb? Yes. Its also how the world works.
Finally, crucially, the idea that forcing Bretonnia to pay is a punishment completely misunderstands how trade works. When person X trades good A to person Y for good B, X values B more than X values A, and Y values A more than Y values B. So both sides benefit. It creates a win-win scenario, as both X and Y are better off than before.
For example, Bretonnia's next Errantry War targeting the retaking of a Karak costs Bretonnia little, as it's really just asking for a shift in targets. But to the dwarves, it's incredibly valuable. Providing Waystone secrets to Bretonnia costs us little (providing them with waystones themselves costs us a fair bit though), but helps Bretonnia immeasurably more.