hmm under the new system, gold is based of of population. i think 10 ppl give 1 gold per turn, multiplied by modifiers. Markets give you +25% fairly quick.
1 food grants a hut for 50 population in kingdom, and that gets bigger later with better tech. (wait i think its 25 food for a hut and 50 for a house)
assuming you build the beacon of hope for example, your prestige will fill that population count fairly quick, perhaps by 50 turns or so. so lets say by turn 75 you have maybe 100 population. that gives a base 10 gold tax rate, again this is also modified by tech. This sovereign skill has 2 tech modifiers that increase its usefulness. I don't think any other sovereign trait has a modifier like this.
so yeah, i think it is underpriced if it works that way.
For comparison, +1 tech give us a one time bonus of a few materials. However, it saves us 1 gold per turn and a population count of 5 but it is not affected by tech modifiers as far as i know. (although that +1 tech lets us invest it into any kind of upgrade you want, but it's a less of bonus than what the food grants us)
The +1 gildar in the new system helps us less than before, however it is useful for replacing some of the taxes, or paying the upkeep of some early army units. It used to be flexible enough to cover the maintenance cost for several buildings but those costs have changed and this skill has not. I think it would be pretty cool for a sovereign or hero to be able to get a level up ability that upgrades this skill. It can start at +1, which you can buy at creation or with a level up, then another level up can change it into a % modifier for a city, than maybe later a % modifier for the kingdom, perhaps the kingdom wide one is too powerful, but it is merely an example. The feat system for heroes has so much potential i'm sad it wasn't implemented and i will root for it every chance i get to make heroes more interesting in this game.