You're not going to get much by way of reductions in airfare by traveling double.. at least, I never have. Check out Air New Zealand's web site and subscribe to their newsletter to get the latest web specials. They have some great deals from the US west coast, even during peak and shoulder season.
As far as tours go, there's the ubiquitous Kiwi Experience
, which is popular with the young-and-partying set. Magic Bus
offers a similar bus-based experience, and I believe ends up costing a bit less than KE. The one that came most recommended by hostel managers I talked to in NZ was Stray
. Again, they offer the same type of deal, but they purportedly get more up-close and more off-the-path than the others.
Either way, you'll love NZ as a solo traveler. In fact, I think I met more solo travelers in NZ than I did anywhere else, and I wasn't even on any sort of tour. One thing you can do is look at the itineraries offered by the aforementioned companies and put together your own thing, if you're alright with setting off on your own.