This is a must-read: Nicola Morgan on why should authors be paid? Because it’s hard, skilled work and professionals are paid http://www.thebookseller.com/blogs/fair-pay-authors-306276 …
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.