I have two weeks of data hinting that the net flow of autogenerated guild funds minus guild repairs is in the red.
I just occasionally wrote down the guild bank balance and checked the money log for non-repair related withdrawals/deposits. The money log sucks and does not go back very far, so there's probably some deposits/withdrawals that I have not accounted for.
If you don't count people giving/taking gold from the bank for things other than repairs (such as goodness of their heart, or raid drops sold for cash), it looks like the guild bank has lost about 20k over the last two weeks.
We've had 4 raids in that time - Two Long Island and Two Jagger.
In order to keep guild-funded repairs self-sustaining, each raid (Jagger, Long Island, and Stoli) should probably donate some gold generated from selling raid loots, when they can. The reason I say raids should donate is that they chew up a lion's share of repair dollars each week. They are the best geared players wiping the most in a short period of time. They are also capable of affording it, since a typical week seems to drop 10-20k worth of auctionable loot.
This is by no means a mandated tax on raid, y'all can choose to donate or not to keep the guild repairs self-sustaining. A good fraction to donate is ~25% of the cash generated from selling raid BOE drops that people in the raid don't need. I think this would come out to enough in the long run to cancel out the red ink.
Obviously guild repairs will have to stop someday if they are not self-sustaining. Blizzard will enforce that