Balance over monster loot and buying gold
I think you should balance loot from monsters with buying gold for real cash. So it'll more of "I like this game, so I'll support starving dev" thing rather than "I need more blocks for my house and I don't like to kill lawyercats for eternity, so I'll just buy gold" type of deal, because I find latter scenario not fair to the players. Mind you Humble Bundle, despite the fact it was pay what you want deal raised WAY over 9000 :) of income, even though you theoretically could just paid one cent.
And while I know that for MMO pay-what-you-want is a no-go (then again, no one ever tried so it might work out), balancing gold and monster loot is fair to the players so they won't be forced to buy more gold to build nice buildings in reasonable amount of time.
Given some of the epic structures built by players who have contributed no cash, I think the balance is pretty good at current.