I have done some research for a question similar to this (but fundamentally different from it) on SF&F, but I couldn't come up with any specific numbers.
The Screen Actors Guild requires that actors be paid for their time, and the rates depend on:
Whether the part includes lines
How many days the actor is on set
Whether the actor is prominent in his or her scenes
And a number of other factors.
Stan Lee appears in most MCU films, usually speaking a line or two, and always in a prominent place onscreen. I'm not sure whether he's an SAG member.
Do we know how much he is paid for these cameos, assuming (as it seems safe to do) that he is paid at all?
If such information is unavailable, can we hypothesize about how much a person should make in roles along these?