Maybe I got the wrong impression but it sounds as if you are expecting the shop to diagnose and then you want to buy and replace the part yourself. There are few shops that can operate in this mode.
If they are diagnosing and replacing parts themselves, then a part that does not fix the problem can somehow be adjusted in the bill.
The shops are not in business to hand out free or low priced diagnostic information. They are in business to repair your car and make enough profit to stay in business.
Had the responsibility to "fix the problem" been put in the hands of the shop, they would in all liklihood have it done and the final bill would have been less than what you have spent so far.
I am all for DIY repair for those who have the tools, equipment and time, but there are just some situations where you are better off "handing it over."
The difficulty for me, and it seems for you as well, is knowing WHEN that time is.
Not all shops operate in the same manner. I agree that it is difficult to find a competent and fair shop. If I only lived in Gainesville, Florida I'm sure my hands would be cleaner with many fewer cuts and scabs. But once you FIND a good competent shop that you can trust, you need to work with them so that you can get a fair price AND they can make a profit.