What is a generous/stingy Tip to a Porter, Stablehand, or Potboy in your Setting?

Normally any RPG system, will have a table of different lifestyles and the amount of gold / resources is needed per day / month to achieve that lifestyle. If you assume, that porters, stablehands and potboys are poor, you can extrapolate from that. It is thus more system dependent and not so much setting dependent.

For example the lifestyle expenses for D&D 5th Edition, looks something like this:

Lifestyle	Price/Day
Wretched	-
Squalid	1 sp
Poor	2 sp
Modest	1 gp
Comfortable	2 gp
Wealthy	4 gp
Aristocratic	10 gp minimum

Having said that, tipping is not something that is needed. The question looks to me very American focused, as there the tipping culture is something out of necessity and obligation, and not about appreciation. Considering as well, that tipping was more done by nobility during feudal times, it is more than likely that there will be no tipping in general.

It also is highly depending on the situation. If they want some information maybe, the players will more likely give some money to strengthen the deal. At that point you pay for the service, it is no longer a tip. Furthermore it depends on their own monetary situation. If the players are very rich, they are more likely to give a "big tip", because for them it is just a fraction of their money.

It is also fun to let the players guess what might be a good amount of money. I had it one time, were they discussed this for a solid 30 minutes on how much money to give for some information, while the NPC was waiting besides them. They discussed themselves down to a lesser amount than initially proposed, so the NPC gave them some shitty information and went away, pissed they didn't gave him more money.

So let players talk and use every word against them...