From memory the ferry is about $20-50 per person depending on the fare, and around $120 for a car to go across. The companies are Bluebridge and Interislander. Not much difference in either of them and they leave from the same place.

I haven't been to Napier but I hear it is lovely. It is probably 4-5 hours drive from Wellington.

Also look at bookabach for a place to stay if you are doing overnighters, comparable in price to motels but often a much nicer and more private setting