The Tech update "Increase Sales Tax" is supposed to give a 30% bump to money generation, right? As far as I can tell what it actually gives is closer to 2%.

The math is done on some base value, without any other modifiers. In other words, % modifiers do not multiply, they are just summed up and then applied altogether.

I don't think it's a bug.
Your money income consists of territory incomes and bonus incomes.
Increase Sales Tax only affects territory incomes, not bonus incomes.
Most of your income comes from bonuses. For example, I'm playing on the AD 1045 map. The Principality of Serbia bonus gives me 58 money per second. It consists of 2 territories, which gives me 1 money per second and 2 money per second respectively, which is almost nothing compared to the bonus itself.
A +30% boost to your territory income shall naturally have almost no effect on your total income.

I honestly didn't realize that single territories gave you any income. I thought bonuses were the whole deal.
If it is as you say though, then the upgrade itself seems silly. The ressource cost to doing it is very high, and it does almost nothing. But I guess that's more of a balance thing than a bug...

idk what level you are on, but especially for the later levels, I find the cost of the territory income tech is not that much compared to other techs in the level

Currently playing Far Land