Just checked the in-room guest brochure. It states SBD $100.00 one way which converts to about AUD$14.00-$17.00. It doesn't clarify if per person.
Amos is correct... a taxi is about the same price and make sure you negotiate the price before you go.
Hope this helps.
Not free... A small surcharge, or you can get a taxi from the airport- $100SBD
Get an answer from Heritage Park Hotel Honiara staff and past guests.