Usually most shops will charge you the book value when it takes less time and then charge more if it takes longer (or sometimes the same as the book value).
The worst example for me, while not based on book value, was when I brought some control arms into Can Tire (only thing open that day) to get some bushings pushed in. Guy tells me it could take awhile so they'll charge me an hour of labour. Ok fine, I guess. Seems high just to push some bushings in but I was stuck so I couldn't say no. Literally 30 seconds after I hand over the control arms to the guy and turn around to go browse while I wait, I get called on the PA system. I go back and say, "So do I still have to pay for a full hour? That took less than a minute..." Guy goes, "Yep. We gotta pay for the machine somehow."
That was probably the first sign I shouldn't shop for car repairs or parts at Can Tire. There were many more incidents since and now I won't go near the place unless I just need to buy some cheap crap not related to cars. If you want a story about how a store with the word "tire" in it fails epically at tires, shoot me a PM.