People like a set price, so that they can know if they can afford the project, and what amount they should be preparing. Professionals working by the project do not like set prices, because they often assume best outcomes, which happen less than always.
I just had a project this week where I asked my layout person for a quote, which he provided, and added on my third – 35% – as my fee. The work eventually needed additional typing and diacritics, or nikkud. The layout person also had underestimated. The client was doing this on behalf of someone else and couldn’t pay any more than had been agreed upon….
The suggested solution is to add a percentage. The numbers I have seen is to add 50% to a project, so that one would quote 300 for a project estimated as 200. At a minimum, one should add 20%.
Effectively, someone asking for a set fee is requesting that he be covered against things taking longer, and is asking the freelancer to take on this risk. Nothing wrong with that, but it has a price, and that is the 50% buffer added to the quote.