Cost was £1005 for the steering wheel (enter the part numbers into
www.mercedes-benz-parts.co.uk for a price), so not cheap.
Paddles are a lot nicer to use and feel better (nice aluminium metal feel) in my opinion than the plastic buttons, you can also see them rather than having to feel them on the back of the steering wheel (although they work exactly the same). I would prefer them to click a little when pressed as they are very quiet and quite soft so you are not always sure you have pressed them, if you see what i mean.
I also had to buy a torque wrench to be able to tighten the steering wheel up to 80nm pressure as i did not have one.
I think the non AMG steering wheel is cheaper (about £830) but has the plastic 2006 smaller paddles with + and - rather than the larger aluminium paddles with UP and DOWN on them (people seem to sell these on ebay for around £140 but like danslk says not sure how they can sell these so cheap).