The default setting for shields to gold is 4 to 1, so 12 shields will give you three additional gold. You can check that by going to the Editor and checking on the General Settings.
When you discover the "Economics" technology, that rate gets improved to "2 to 1". (So 12 shields would then be converted to 6 additional gold.)
@robbus: to make a long explanation short: to understand the Despo penalty, just take a look at one of those irrigated cow tiles in the two screenshots. In the Despotism screenshot it provides 4 food, while in the Republic screenshot it provides 5 food.
The full definition of Despo penalty is: "Every tile that produces 3 or more food/shields/commerce, gets 1 subtracted."
Food is of course the resource, where this is most prominently felt... (It's the basic rule behind the rule of thumb "irrigate brown, mine green", as irrigating grassland ("green") has zero effect under Despotism: unimproved grassland provides 2 food, by irrigation you increase that to 3, and the Despo penalty reduces it to 2 again...

The AI apparently doesn't know this: just take a look at Amsterdam in our current succession game: the Dutch have irrigated 4 grassland tiles while still in Despotism. Lots of wasted worker turns... If they had built mines instead, they would now have +4 shields -- and we wouldn't have to wait so long for "our" Lighthouse...

)
But it also effects the other two resources, gold and shields. For example, the Despo penalty is the reason, why you don't mine hills under Despotism: a hill provides 1 shield, the mine increases that to 3, and the Despo penalty reduces it to 2. So the mine gives you only 1 extra shield, but it takes
12 turns to construct. You can build a mine on flatland (grassland or plains) in only
6 turns, and it also gives you 1 extra shield. So why bother with the hills...
Therefore an experienced player will first improve all the flatland tiles and only turn towards hills & mountains, if a) all flatland tiles are done and/or b) if Republic has been reached. (The exception is of course an important luxury or strategic resource that needs to be hooked up early, e.g. wine or iron on a hill or gems on a mountain, etc.)