Rear Brake Cylinder cost of repair

Not necessarily. Usually when the rear brake cylinders fail, brake fluid leaks out of them (if they weren't leaking, why sould someone replace them??) gets on the brake shoes and those are replaced at the time. To do that properly, the brake drums need to be machined, all adding to the cost. Then there could be wheel bearing issues to deal with. If nothing else at all was done besides replacing the rear cylinders, then the cost seems high, maybe, but I don't see the job going like that.

You could have done it yourself for about 20 dollars and a pc of sandpaper.