If you just want a two way air ride kit then raven do a kit fitted for £1700 and bagyard do one for £2500ish well thats rough prices ive had for a mk4 golf you can buy kits in from the states for like £500 and fit them your self but you need to run all the lines etc.
Hydraulics are way more tho. There are a few guys that come over to the cov mega meet from unity low riders and boy they get them things bouncing with the snoop doggy doggy blasting out the windows