The springs are mounted on the rear Macpherson struts; the struts must be removed and disassembled to replace a broken spring. Springs usually break because of worn out struts that allow the suspension to move more than designed; if the struts do not absorb road impacts, the force is transmitted to the springs instead and this can exceed the design tolerances the springs are designed to handle. Also, the parts on the other side of the car are the same age, and likely in similar condition.
There are several ways to approach a repair of this type. The professional way to repair this condition is to replace both rear struts, both rear springs, and then perform a wheel alignment to make sure everything is adjusted correctly. this will result in a repair that restores the car to as-new operating condition so that you will experience normal handling, normal tire wear, and no problems down the road.
A shop that does less than professional work may offer you a discount job that involves only replacing the broken spring; that is another approach that costs less, but may result in abnormal handling and tire wear issues.
To repair a broken spring correctly, you will need the following:
(1) AC-Delco Rear Strut, part # 503375 $ 105.05
(1) AC-Delco Rear Strut, part #505376 $ 112.62
(1) AC-Delco Rear Spring Set, #45H3121 $ 162.49
(1) Wheel alignment, estimated cost $ 60.00
labor to replace rear struts & springs (1.9 hours) $ 142.50
Total (excluding tax) $582.66
You have to keep in mind this is only an estimate; prices of parts and hourly labor do vary somewhat in different parts of the country. Also, there may be other worn or damaged components that you are not aware of that may need to be replaced as well to repair the vehicle. This estimate is based on the information you have provided.