To pay or not to pay, that is the question

What is your goal?

I don't think there's a right or wrong answer other than if you do prime yourself, definitely work very closely with the painter so it's all compatible product etc. I doubt you'll save much by spraying primer yourself though, that's easy and quick, but you could save a lot by doing all the bodywork and block work yourself so long as you're prepared for the outcome. Either way, have them do the final finish blocking to get it just right, but of course if you've muffed it to that point, they won't be able to fix it at that stage without digging in and maybe even costing you more $ than it would have to begin with!