Sorry, but youre wrong. Ive been on both sides of this work as an engineer determining repair time and as a tech manually validating every task within a technical manual. Book time has a significant safety factor built in so that the average tech should be able to complete the task in 70-90% of the time given, and does account for rust, broken bolts, and other issues.
Estimates are that simply bc even the best technician cant realistically predict everything necessary to repair a vehicle under all circumstances, not to cover technician error in the repair process itself. IMO the OP got screwed.
I hate to bust your pride, but I am not wrong. I have labor guides (Alldata, Chiltons) that are digital (current), and labor guides that are physical books from years back. The labor times have not changed. If that truck was brand new, the exhaust studs would not be snapping off flush in the head, they do when it is several years old, however, the book time has no been adjusted for age even in current guides. I have had this discussion before with my regional sales manager from AllData who previously held a publishing position with the same company.
As a shop owner, I am not financially responsible for any of the repair of a customers vehicle. Example, our shop did brake lines on a 2001 Silverado w/ ABS the other week. These full size GM trucks are notorious for rotten lines, and are one of the most difficult vehicles to replace the lines in due to the placement of the abs on the frame rail (the ABS to rear line being the most difficult). I quoted the customer $750+ tax to replace all the lines, and use my $4,000 investment (scanner) to bleed them (AKA, DIYer generally cannot do this job). There is no book time for brake lines.
During the bleeding process, BOTH of the rear caliper bleeding screws broke flush. This is after we heated them, and they would not budge. We removed the calipers (extra, unforseen labor). I purchased a kit ($120+) that fits my air hammer ($310). This kit has a pin that is inserted in the air hammer, then inserted in the bleeder screw. You activate the air hammer, and the vibration generally loosens the rust bond on the bleeder and you can turn it on with a wrench. It destroys the bleeder, but saves the caliper. It did not save this caliper. The bleeders still busted. Needs new calipers. $45 a side.
As a shop owner, I certainly do not eat this part. They are not needing replaced due to our negligence. My mechanic did everything possible to remove the bleeders, and they still snapped. The customers bill was charged $150 extra. $90 for the calipers (45 a piece) and $60 for the R&R of the calipers and the shop time spent trying to remove them. To sum it up, while we were waiting 90 minutes for the calipers to arrive, my mechanic is sitting still while I am still paying him. There is nothing for him to do. His lift has a customers rusted our truck on it. Our labor rate is $60/hr and I book at a minimum 8 hours of work a day for our mechanics. This customers problem cost the shop $90 just in idle time.
The customers bill was $900. They understood the issue, and signed an agreement before hand that we are not responsible for ANY broken bolts or part due to removal. I charge the hourly rate to extract broken bolts and so does every other profitable shop. Sure the customer can complain about the extra charge, but the difference is in 10 years my shop will still be around to serve customers, and the shops managing incorrectly will not.
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Some jobs, service information and labor times are just an engineer's crack dream.