Hi Fiona!
Sorry I cant give you any costs as my last trip was six years ago and just can't remember.
We were based at the Silver Beach hotel on the east coast in Trou D' Eau Douce ,would have loved to stay at the Le Touussrok but couldnt afford it. A boat trip to the Ile aux Cerfs is a good idea from here,
As Mauritius isnt a large island, with the help of the Lonely Planet guide we planned our own trips and hired a taxi, north south and central a day each.We found this better than the organised trips as we could go to just where we wanted and make stops for photos when we saw something interesting. The Hotels have the same system as in India with their own taxis, and they will also want to take you shopping! If possible try to find a local taxi outside the Hotel it will be much cheaper.We used the local buses for trips to Centre De Flacq ( just Flacq to the locals and very indian with good local market ) from here you can get connections to the rest of the island
My personal tips--
Grand Bassin- beautiful location and Hindu Temple
The Hindu Temple at Poste De Flacq, another lovely setting
Black River Gorges and Tamarin Falls for the views
The Botanical Gardens pamplemousses
ps--- They say its raining somewhere on Mauritius every day of the year! so be prepared for some showers, it was the unpredictable weather that put us off going again after two visits