I will explain the math to arrive to this number in the second half of this post (I know it's not everyone's cup of tea) and I'll just skip ahead to the result for the moment
So: The bigger the city/town/barony/village/community etc, the bigger the reward it can afford to offer. Of course, two communities could have the exact same number of citizens but have different level of wealth, but this is a baseline. You also have to consider the danger/importance of the task. If a goblin is stealing sheep, you don't offer 10 000 gp. Low importance/danger/priority tasks are "copper" level. Serious issues are silver. Dire ones are gold.
So take the number of people in the town, multiply by 2, then give that amount of coins in reward of the suitable level. Here are a few examples:
The road leading to a village of 400 people has been afflicted by a band of brigands, severely impeding trade. Task level serious (silver): 400 X 2 = 800 sp = 80 gp to take on the brigands. This isn't a lot of money, but the village *doesn't* have a lot of money, and the brigands haven't been attacking the village directly.
A town of 3000 people, poorly fortified. An aggressive tribe of orc is in the area and scouts indicate that it will probably raid the town, killing several people and inflicting major damage. at best (they could burn it to the ground...) Task level: dire (gold) so 3000 X 2 = 6000 gp to deal with the orcish threat. The mayor is opening the city coffers to save his town.
A city of 50 000 people. Groups of convics are responsible to maintain the sewers, but some have disappeared. City guards have been unable to find anything. Divination magic indicates that the prisoners have not escaped but have been slain. City officials want to keep this quiet before panic follows. Task level low: 50 000 x 2 = 100 000 cp = 1000 gp
You will note that this means that the reward for a low threat is quite large for a large city. This doesn't meant they are throwing their money around. Rather, smaller threats are simply dealt by the people the city have on permanent employ. The city would simply send a company of horsemen to sweep the road clear of brigands for example. It's only when their own resources are insufficient/inadequate that they need to hire "specialists" - the PCs.
I'll note that these reward represent a ceiling for rewards, and that a town may offer *less* money for a task to be accomplished. But it also means that a hamlet of 100 people can't offer 5000 gp for the return of a kidnapped child - they just can't afford to.
I also note that these numbers are for 5e. In older editions of D&D I believe that the laborer's salary was 1 sp/day, not two, so half the numbers. For warhammer or the GLOG... I hadn't done the math yet, but using the above principles it shouldn't be too hard.
Lastly, these rewards could mess up game systems where the XP reward is based on gold found/awarded, or games where magical items are easily purchased, so gold = power (I'm looking at you pathfinder), so it's something you should keep in mind.
So how are those numbers derived? Well, how does a town make money? Taxes! How much money is there to tax? The 5e PHB on page 157 gives us a good idea of people's income. So first we have to divide the people into income slices. Here are the numbers I came up with. They are based on the assumption that many people were simple peasants/laborers and thus poor. This is also linked to my "economic yardstick" concept - in other words, what the PCs can be paid is based on what the standards of living are.
20% non tax payers: this include people too poor to pay taxes, criminals, children etc
50% poor: their income is 2 sp/day
20% modest: income is 1 gp/day
8% comfortable: 2 gp/day
1% wealthy: 4 gp/day
1% aristocratic: 10 gp/day.
Using a spreadsheet you can easily calculate the population's total income per year, and then figure out the average income per citizen. I did this and calculated that it is about 220 gp/citizen year.
So how much of this income can be taxed by the city? Well... not as much as you may think. The people are also paying religious tax *and* taxes going to the king/emperor/duchess whatever. So the mayor/baroness/city council can't take too much. I've estimated this to be 10%.
So this mean that the city yearly budget is 22 gp/citizen. A city of 10 000 people therefore has 22 000 gp to run the city per year. I decided to round the numbers a bit to get 20 gp/citizen
A low threat would warrant a response representing 0.1% of the yearly budget. A wise mayor has probably money set aside for such events, which happen semi-regularly. that means 2 copper piece per citizen
A serious threat warrants a serious response, and 1% of the budget get devoted to this grave problem. That means 2 sp/citizen.
A dire threat is near catastrophic, and the town opens the coffers in the hope to lure powerful hero to save them! 10% of the budget - all that can be spared really - is devoted to this problem. Ie 2 gp/citizen.
There are threat levels which are quite frankly catastrophic, and deserve an even bigger response. However, at this point the sheer scope of the threat becomes more important than any rewards. The PCs will either go "The demon Shlub Megawrath from beyond the star is melting the bones of your children?!? This evil shall not stand!!!" .... or they will go "So you managed to anger 50 dragons eh? Thanks for letting us know. We have an urgent... appointment... in that other kingdom we have to go to... good luck!"
(this post is based on an old EN World post)