I've said it before and I think most of us on here try to help the local shops as much as possible, but up to %20 is what I personally find reasonable. I think with the dealerships it has more to do with invoking negative emotions we've had with dealers that have ripped us off in the past or been completely incompetent and its left a bad taste that affects all future dealings and with all dealers.
A good example: We recently got back into our house in Mississippi. Turns out our previous tenants had sabotaged our pool liner. We had water getting behind the liner in a heavy fashion. I called the pool company I've used time and again and had faith in. The owner himself dove it 3 times and couldn't find the leaks. He had people pressure testing pipes, resealing lights and stairs, and spending several hours working on it over the course of 3 weeks. We finally drained the pool as a last resort before busting concrete and found the leaks. He was profusely apologetic for missing them during the dives. But here's where the example is. During all that time, I never once asked, "what's this costing me?" I knew he'd charge a fair price for fair work. At the same time our salt water cell had gone out so I asked him what his price was for it (a 3 year part, that usually lasts 5). His price was $500. I could get the exact same one for $400 on Amazon. A %25 increase. I bought from him. Turns out 3 weeks of work (probably 100 man hours) and a fixed pool cost me $1100 including the cell. He didn't charge for the dives, the pressure tests, etc. just the actual things that resulted in a fix. That is exactly why I'll support my local small shop rather than Amazon when I can. Second of my long winded points is, if this had been a similar job with the New Orleans triumph shop, my reaction would've been completely different as my dealings with them in the past have proven them to be lyers, cheaters, incompetent and rip offs and I will never again do anything extra to support them or give them the benefit of the doubt!
