The entry makes no mention whatsoever of how much cash the political house might have, just that cash is the kind of thing such a house has on hand to spend as needed. Their activities require the ability to sometimes spend money without a lot of warning, which means they need to have some portion of their funds ready to dispense at a moment’s notice. That could include dispensing it to the PCs.
The amount of cash they would have would depend on a number of factors: their size, the size of the city they’re in, their social class, and so on. The Community Wealth sidebar on page 46 describes this in more detail, though it is still well short of “a given noble house should have ready cash equivalent to the wealth-by-level of an Xth-level NPC” kind of detail. At best it will tell you how much cash the nobles in a district are expected to have, in total, and then you have to figure out—for yourself, on your own, as far as I can tell—how big this particular house is and how large a slice of the total assets it controls.
Note that a particularly big noble house may easily have far, far, more money than a 6th-level PC, but that doesn’t mean they’re going to just hand it all to the PCs. They may have many, many other projects going on that also require funding. So that may influence things; depending on how detailed you want to get, you could even make up what some of those projects are, what their priorities are, and so on. More detail could include NPCs in the house working on different projects, all trying to secure more funding for their own projects or preferences, etc. That kinds of things that influence real-life decisions made about funding.
But overall, I would play house money as a more narrative feature: the players have to convince their patrons that they need money for some specific, particular expenditure, and get the funds to do exactly that thing. Spending it on something else is, basically, theft, so the players should know that the house won’t take that sitting down. When I read these rules, I imagine the scene in Inception when they’re talking with their patron/customer, Mr. Saito, about bribing all the staff and personnel on their target’s flight, and he turns around saying, “I just purchased the airline. It seemed... neater.” Or in Ocean’s 13, when unrest at a factory was threatening their plan, and they learn the workers’ demands total $36,000 (a paltry sum compared to their total operations) and Danny Ocean exclaims “we’ll write them a check!” only to look sheepishly over at their patron to confirm that since it wasn’t his money. That is the kind of relationship that PCs should have to their patrons’ money.