Depends on how complex the job is, but most of my recent UHF DTT antenna installs end up coming in around $150-$250. This includes antenna, mast, new cabling and testing with a DVB-T meter. Would be much less if current setup is ok and just an antenna is needed to be added to a current mast and cable setup, however If the cabling is readily replaced I recommed doing so with quad shield for DTT. That said I have had no issues on systems using exsisting Sky Sat approved duobond RG6.
If you live between Palmerston North and Wellington (inclusive) sure. I think there are plenty of installers in your area (south Auckland) yellow pages should help. Also try Paul 021 681371 or 09 5273848, firstname.lastname@example.org, he covers Panmua, Mt Wellington, Remuera, East Auck