Ciao Daniela,
It's strange they haven't gotten back to you yet, know Berta is pretty good about replying. Give her another day or two, could be she's behind....
I don't really know info on the prices, but I know she'll give you a good price for staying longer. They do by the night, so 5 days is definitely doable without a problem.
As far as CT goes, was afraid you were planning a day trip, heading back to SG at the end of the day... which would be hurried. CT is definitely very nice, although beach area is limited and in July you'll want that. There are more options around Monte Argentario. Also CT has very limited cover, so if July turns out hot, you'll be sweltering. I normally don't recommend CT in the summer, since it is also very crowded as space is limited, much like Venice.
So I'd be more for recommending Monte Argentario, especially since you'll also be in Sorrento later on... a good mix of two different areas.
You could do a night in CT, so that you still see the five small towns, then the nights in SG (great base for seeing Siena and the Val d'Orcia, as well as Chianti and Florence), then move on down to Monte Argentario that way you move down as you head south...