It works. It's a concept known as teamwork, which is probably alien to many who are used to working alone. Basically he treats working for/with me like employment. Turn up, do stuff, go home. I tell him what to build, ask him what he wants where, I make it all happen. Buckets of sand and cement are always lined up next to the mixer for him.
I realise it's not for everyone but it works well. A sort of DIY/get someone in hybrid.
I'm absolutely certain that I'd pay much more if I got someone to price the job up in advance, as then they'd include a load of contingency that they probably wouldn't need which would just become extra profit. Plus they'll include the time spent measuring up and calculating it all. They'll assume the worst case at every turn. If you want someone else to take all the risk away from you then you need to pay them money for doing this.