So I've been pondering this, and for certain situations (particularly the buying of ships) it seems to make sense that a Dynasty might want/need to "burn" PF to make purchase rolls that would otherwise be out of their reach. This might be handing over such staggering amounts of currency that even a Rogue Trader dynasty is out of pocket, or might be trading in kind by handing over trade concessions; renowned artifacts or even whole colonies.
The thought is, how much PF for how much of a bonus? My thought was that, after all rolls are completed, the players may improve their roll by +5 for every point of PF that they are willing to give up afterwards. This (hopefully) makes it tempting when that Best Craftsmanship Inferno Pistol is just out from what you managed to roll after the commerce checks and fate point expenditure, and also means that the +60 you would need to get the fully outfitted light cruiser is enough to hammer dynastic resources without completely ruining them.