SES Light flashing for P0306 Error

I have a 2000 Chevy Blazer and had the SES Light flashing. Using my scanner I found that it was a P0306 Error for misfire in the #6 cylinder. I heard bad things about this shop but being I just moved and that was several years ago that I heard the bad reviews, they were closer (7 miles) than my trusted mechanic (35 miles) I took it there since I was leary of that drive. They billed me $100 for the diagnostic and said that I needed a new CPI Unit for just over $1,000. I couldnt do it at the time so I waited on the repair. When I went to leave, my vehicle wouldnt even start now, found out that they did not properly hook my battery back up. After tightening the battery I went home. I made some calls to price shop the repair costs. I called my trusted shop and another location I have gone to before. They both quoted me over $250 less for the repair but both stated they had serious reservations with that diagnosis. I just brought my vehicle up to my trusted dealership a few days a go to have them look at it. They did the diagnosis and said that it was 1 bad spark plug and 1 bad injector. They also recommended having a general fuel system cleaning to get the other injectors clean before they clog up too. For the new spark plug, new injector, fuel system cleaning AND diagnosis I only paid $540. Boy I wish I had my $100 back for that bogus diagnosis. Stay away from this dealer as they will give you a faulty diagnosis for repairs that arent needed or do way more than what is needed (like replace a whole system when only 1 part is needed).