According to this post, the NISMO spoiler(not the S-tune version), is very expensive - over $2400 plus paint... but it doesn't need any new holes drilled.... see jazz.41's entries...
Nismo rear deck spoiler on 370Z sport coupe
However, IMO, it doesn't look right without the NISMO rear bumper as well - looks too tacked on - unless you do a full NISMO body conversion, i think just the spoiler will look 'uncool' - just my opinion.
Picture credits to jazz.41: