The springs themselves are around £30-40ish each for a small car. Obviously the amount of work vaires from car to car, but as a guide on a vw polo it took around 45 mins/1 hour per side to get the strut assembly out, strip it, and put it all back. So you're looking at maybe £80-90 for an hours work - expensive for a local garage, and maybe a little on the high side for a main dealer. Depends if they replaced any of the mounts etc at the same time - they tend to become mishapen over time as the car's weight is always sitting on them.
rover are i con, it cost me £30 for them to say "we suspect the ECU on your alarm is broke and requires further invertigation!!" what a con. to think, my dad's friend changed the rear axel on my rover for only £150, for labour and a 2nd hand rear axel and a friend told me it would cost upto £800!!!