I paid €100 for a new key + immobiliser keyfob gizmo + "tuning" of same, for a 1998 Primera, though admittedly this was about five years ago.
However I've since noticed that the fobs can often be found on eBay.co.uk - check all the serial numbers and details on the back and have a search. Should be very small money. Then you still need to get the key copied (guessing €40 max) and the keyfob "tuned" by the dealer (dunno, but it has to be cheaper). Ring and check in advance that they'll do this though.