Right now it costs 10 gold PER hour to hold a keep in Tier 4. I've seen many small or medium sized guilds discussing 15, 20, 25% tax rates, and their guild populations are crying out to lower them as they say the rates are too high.
I can understand complaining when you might have need for that gold, but right now I'm not sure there is much need for gold in the game other than these combat focuses. So a 15, 20, 25, or even 50% tax rate to hold keeps at higher Tiers may not only be necessary but justified.
Although I see potential exploitation in the system (what system doesn't have the potential on some level), I think the developers should offer a percentage tax rate depending upon the guild strength and/or population count at the time.