It is free, and in fact the entire island is free to visit. The only cost is the ferry ride over. The monastery is pretty much open air and you are free to wander through on your own. There is not much in the way of information posted like we are probably more used to here, so if you are interested in the history, I suggest reading up beforehand.

The winery is only available to tour on the 1st Friday of each month. The is a shop at the monastery where you can purchase bottles of wine. We still higher recommend going. We were just there today.

We took the 11AM ferry over to the island (takes about 25 minutes), then we walked around the perimeter, visited the fortress (took about 25 minutes) and ate our picnic lunch before catching the 1:30pm ferry back (the ferry departs back for Cannes at the bottom of every hour.

The water taxi runs throughout the year, costs around €15 return for an adult and takes 15 mins each way. It leaves from the end of the Quay adjacent to Laubeuf car park on the western side of Cannes port. The schedule varies according to time of year but runs up to 10 times a day in the Summer. There's a website (in English) - just Google "water taxi St Honorat Cannes".