I am in an unqiue position that clients seek me out based on my experience to solve problems, that their inhouse staff can't solve. (software engineering, architecture or specific technologies )
They are often happy and making a bargain letting me do those things at rates of 350 € / hour compared to having to hire / find specific people having that kind of experience and paying them long term . My contracts are usually 6-12 months only.
On the other hand i often negotiate a flat fee for a specific task... For example my last one was solving a very specific computer vision problem for a client after their inhouse project team(s) of 5 people were not able to solve it over the course of 6 months.
It took me about 8 hours in total to solve it, and maybe 40 hours to integrate it with their product/document it.
However i billed them close to 100.000 € for that task, ( pre agreed by them upon successful completion ) and worked bascially a week only.
But compared to 5 people * 6 months * salaries of 100k = 250.000 it was a bargain for them ( not counting overhead costs and the likely hood of further time not solving it )