Well, use your imagination, there are more ways to get gold than through tax days. That said, we looted more than a thousand gold from Astrum, in effect.
Besides, how can you rule out foreign backing from realms to affraid to fight themselves?
Like I said, I can see way of doing it, but personally don't agree with most of them. Its just an opinion. And no, I doubt that on Dwilight, given the military strength the possible realms that would want to oppose Astrum, that such a force could be funded properly from Foreign Backing. I could be wrong, but I have a hard time seeing it.