If you can optimise for a 10% CTR, you might expect an Adsense CPC of $0.30 maybe? That's worth $150/month or $1,800 year. Not sure if that helps you put a value on it or not?
It's always worth throwing some AdSense up on it rather than nothing!
That said, assuming the majority of your audience is from NZ the GKT stats don't look as promising with only $0.55 CPC and medium competition. I'd be surprised if your AdSense CPC was anything higher than $0.20.
That is the problem with most .co.nz domains - they normally have a tenth of the search volumes to begin with, but in addition to that they also have about half the online advertisers and competition of Australian domains.
So the domain itself I don't think hold HUGE value - maybe $100 - $500 to a retail client.
.co.nz hold some decent value for end users but I don't know they they offer much for us developers IMHO.