Firstly, ensure that your suspensions have not become soft by
slowly pressing your car downwards. If it bounces up and down
easily, it is a sign that eminent replacement is required.
You may also want to hear whether there are knocking sounds when
you go over bumps and humps when you drive.
The best thing to do is to actually send the car in to a
reputable workshop for a diagnosis and if it really does need
replacing, please do so!
Otherwise your under carriage might be damaged in the long run
if your suspensions are soft which would result in the
undercarriage of the car hitting against the surface of the road
should it not be able to take the pressure weight of the car as it
goes over road depressions, high humps etc.
For a normal car like a Nissan Sunny, a good estimate would be
around $400 for all suspension struts, including labour.
I am currently using bilstein comfort shocks which retail for
about $600-$700 a pop for all four, inclusive of labour.
Hope this helps mate!
what's your ride? 700 for all 4 kinda cheap.