To summarize: The OP wants to be able to drive into any mechanic's parking lot, interrupt his/her work, not only find out what the car thinks is wrong but get the benefit of the mechanic's experience with that particular car and trouble code, chat for half an hour about things the mechanic couldn't care less about, then drive off to a cheaper mechanic or the auto parts store -- for free. All the while, the mechanic in question is earning no money, while he has a shop full of equipment that's depreciating or accruing rent, and possibly staff that's sitting idle -- all at just the same speed as it would be as if he were charging billable hours.
"Dirty secret" indeed. I think I'd want $200.