Upvote:3
Cargo ships take limited numbers of paying passengers. On average you pay $100 per day you spend on a ship, which makes knowing the total price a bit unpredictable.
cargoshipcruises offers complete intineraries. I am not familiar with the names of Kiribati's port, but I would suggest contacting cargoshipcruises for advice.
Upvote:4
The only person I know who's visited Kiribati by boat is Graham Hughes of "201 countries by land" fame, who managed to blag his way onto the Southern Pearl to Kiribati. But it's a working cargo ship, so don't expect getting permission from owners Neptune Pacific Line to be easy, plus their website also doesn't list service to Kiribati at the moment. Good luck!
Swire and Kyowa also claim to serve Tarawa (Kiribati) occasionally.