Short answer is - it depends.
I have a couple of guys who have done work for me for years. They have their own vans and rounds, I knew them and had seen their work so asked them to do some for me. It suits them and it suits me, so the arrangement lasts. If it works for both of you, then it will last, if it only works for one of you, it won't.
So in my view you can't expect to make a big percentage for a good job done by someone else and have it last a long time. They are either going to start doing a poor job or get fed up and drop the work.
I have always taken the view that if everyone makes money then that's fair, people don't feel taken advantage of, and the situation will work longer term.
If they are good, and you can trust them, then look after them, pay them promptly and usually it'll work out, people aren't stupid so you should always remember they have a choice.