Kind of a trade secret I think
In my experience if everyone makes money the situation will work and continue. If you look after people and pay them fairly and on time they will be far more likely to do a good job, and you get to be a nice guy.
We have all done work for people who pay late or badly, you don't bust a gut to do it and as soon as you can you drop it
I have a few jobs I sub out, I don't make much on them but the people who do them are really good and I don't have to worry about them.
If you take liberties why should a self employed person with an ounce of common sense carry on working for you, as soon as they can they will bin it.
So no answer really but you get my point.