for an OEM set you're looking at best part of a grand, each cluster alone is just under £400 and the HID bulbs are about £80 each, none of the other bulbs are the same as the standard lights either (this makes buying spares a chore), you also need the washer jet (if you've got the winter pack you're laughing)
you can get after market "conversions" which sit in your existing cluster, however these are 100% illegal (regardless of E marks, kite marks) as the law is quite clear when it says you can fit them, but you have to then fit a suitable cluster as they have to be focused properly and auto level.
IMO they are well worth the money as you well defined light pattern, the light is easier on the eye, you can actually see on country lanes without full beam and they look the shiz
if you've got a spare grand then go for it, if not get some uprated bulbs.
And to be honest, the alternatives on the market dont look as tidy in my opion as the OEM ones.
I've seen a few round my way that way these and TBH they look sh1t and you can tell that the pattern isn't right