[Moppie]

Struts, in the most commom use of the term, are 3 main suspension parts combined together.

The shock absorber, the spring, and a suspension arm.

All are moving parts, the suspension arm however does not suffer any wear (although there are rubber bushs where it mounts to the car and wheel), the Spring does wear out, but it takes a very long time, and vey high millage, unless your car is over 20 years old its not worth worrying about.
The Shock Absorber however does wear out, and they will usualy be replaced once or twice in the average cars life time (there is one for each wheel).

Some shock absorbers can be rebuilt, usualy they are ones fitted in aftermarket performance applications.
When dealing with orginal equipment shock absorbers (factory fitted) its usualy cheaper to simply replace the unit with a new one.

When the shock absorber is part of a strut the strut needs to be removed, the spring compressed, the shock absorber removed from its centre, a new one fitted, and the whole strut refitted to the car.


Cost is very subjective, it depends where in the world you are, what sort of car you have, avliablity of parts, whether you want factory parts or aftermarket, how honest your mechanic is, ecnomic forces operating in your area, etc etc etc.

Its not an answer you are going to get on the internet, so you will need to get in your car and drive around a few mechanics and get some quotes.
Just make sure you get a quote to replace both sides at once, its very important to keep the cars suspension working as it was designed.
Any mechanic that is prepared to replace only one shock should be avoided.