So, the <calculate> expression doesn't work for buildings. I hope StarDock fixes that.
I've managed to work out another method though, which I think breaks the game less...since my previous method was just pumping way too much Gildar into the economy. The new tax calculation method simply adds a resource multiplier to your Gildar production. The play testing with this new version I've done so far seems much more balanced and fair. There's only 1 tax office now, and it visually upgrades based on your cities population. You need to have a firm grasp on the Economics skill to make use of the office. The offices now also confer a -1 penalty to influence, since who actually wants to pay taxes.
To compensate for the down play of the city based tax offices, I've increased the bonus given by the Kingdom Tax and Revenue Service to 250%. This seems like a lot, but you're only allowed one per Kingdom, and it has to be built in the same city as your palace, which may or may not be your economic capital, so it's not as bad as it seems.
Here's the Download: Click me!
Unrar the Z_TaxOffices v2.xml file to your Program Files\Stardock Games\Elemental\Mods folder.
Unrar the Tiles folder to your My Documents\My Games\Elemental\ folder.
Select yes if asked anything about overwriting...don't worry, unless you've got tiles named Z_TaxOffice_01 etc. you won't actually be overwriting anything.