just for the springs can cost as little as £60 really mate but depends what brand etc
i have rsr titanium ones on mine which are about as expensive as you can get!
I've just t teins on mine with uprate dampers. Really pleased with results. Thumps a bitif you hit a dip in the road about 60-70 but good all round i think. £160 from Camskill.
Before ( dont have a dead side on to show you diference i'm afraid.)
After....
This was prior o it all settling in, so literally just after it was lowered. There is a fair difference but the pics may not show to full extent.