I know nothing about the costs, but if you build, be sure you know about utilities. Are you buying a lot in a subdivision or in town, where the utilities are extant in the road and waiting to be hooked up to your house? Or are you out in the boondocks, where you have to run in power and telephone from a ways off, and you need a well and a septic system? If you do need a well and septic system, is there actually enough water, and has the site been evaluated for its ability to support a septic system?

Also, is the property OK to build on? We currently live in landslide country in California, and often a 40 acre property in the mountains may have only a small area on which it is safe to build. I don't know much of anything about NZ's geology for these purposes, but I recommend that you do your homework. I do know its earthquake country over there.

So many people have been burnt buying property without knowing these things. I would recommend that you ask the utility companies and/or a local agency about utilities, and perhaps also a geotechnical firm, if the property is at all geologically suspect. Don't necessarily trust your realtor or contractor to know this stuff. It isn't their expertise.

Good luck.