As a GC myself I find there is a HUGE dichotomy on what your read as best/fair practices from the perspective of Contractors and RE investors. If you read books and take course to learn how to run a successful remodeling or construction company, they are often the exact opposite principles of what I read in blogs on this site and others about hiring a contractor. I do agree that for all parties a fixed price estimate is best (you need to know how to estimate properly).
To be successful as a GC, things like getting a large deposit, making sure you are always staying ahead on billing and are spending clients money and not your own, marking up all costs at least 20% to cover OH&P, billing appropriately for change orders, not to mention staying up to date on latest building methods and continuing education, are the minimum requirements to last in this business.
Of course you have to keep construction costs as low as possible when you are rehabbing a real estate project. And maybe you will be lucky and find that rare gem of a person who does really good work and will work for peanuts. A rare find indeed.
I have bid on some projects for other investors and find that I'm always higher than what they are used to. My question is; if you are constantly looking for new contractors because the cheap ones are always screwing up, maybe there is a correlation there. A fair price, shared goals and trust is what I look for in my relationships with my subcontractors.