for the price for a flight from mauritius to rodrigues check the homepage of air mauritius. i think it is something about euro 100/150 for a return ticket.
rodrigues is a nice place to stay, it is really calm and quiet, only a very, very few tourists. tourism just is beginning in rodrigues. compared with rodrigues, life in mauritius is rather quick, stressig, ... there are some small hotels, you also can get appartements. it depends what you prefer. anyway, i think it is best to stay in the north, port mathurin or anse aux anglais, it is easier to take the bus from there for travelling around. in the south you just find the "ebony hotel". but as far as i remember, i saw a boat of bluedynamite around this hotel. so it might be the right place for you to stay.