Firestone Complete Auto Care review: Over charge

I had my light engine on and decided to go to Firestone as they were the closest around and I thought such an old company would do a good job. It took forever for them to diagnose the issue and then the $1000 repair was revealed. I had gone there before for a basic inspection, and I had heard that number before for struts replacement that I didn't think I needed and which I didn't do. The intake manifold had to be replaced, this time around. I asked why it couldn't be fixed and they told me immediately no. So, because I'm new in town, I paid $99 for the diagnose (!) and had to come back next day to leave the car for repair. I should have made my search, but at the time I didn't know the name of the part and simple thought it was what it was. When I delivered the car I got the part name and made my research. 12 mechanics in Albuquerque would honor a budget of $220 to $260 to repair the issue! I was furious with myself and even more with the rip off I was undergoing.
When I got to the shop I asked for the part invoice and was shown one for $423. Firestone would add $300 on it as their profit, claiming that it would cover a 12 month warranty on it. A warranty for $300! I was charge $933, because they made me apply for their credit card (28% apr) and was promised a 10% discount. They wanted to give me 5% only. I complained and the manager gave me the 10%. Anyways, I believe I was overcharged by at least $200 (if one considers that from those $300, $100 is enough to pay for the warranty and some profit)