This is a must-read: Nicola Morgan on why should authors be paid? Because it’s hard, skilled work and professionals are paid
Partciularly useful on how you might estimate what to ask for, and how to make exceptions.
I agree completely with the sentiments in that article. One of the things you get told in a business marketing seminar is not to undervalue your products or services, for exactly the reason mentioned in the article: if you do undervalue it, that creates a negative impression with potential buyers.
Promotions relating to book signings are a different matter, as they come under the general heading of "marketing". I.e. they are aimed at improving sales of a current book, and thereby increasing sales. That means that, if they work as expected, there will be a natural increase in earnings from the book, so they will pay for themselves.