Does anyone have any idea of an approximate cost of a car and driver for 4 adults for a day of touring in Tuscany?
If you can drive, with €50 a day (all inclusive) you can rent a compact vehicle and do your own tour.
Don't forget to get an International Drivers License.
I've just been pricing private tours for one day out of Florence and I see prices range from 440 to 600 euros, depending on which tour and the number of passengers.
Check out companies like TuscanTravellers.com or Florencetown.com
I think around 500 Euros total for your group for a full-day tour (10 hours). We used Florence Tour (www.florencetour.com) to visit Volterra, San Gimignano, Monteriggioni, and Siena in one long day in March 2011. If you decide to pursue a private tour vs. the do-it-yourself route, I recommend contacting them for a quote. The company is owned by a husband and wife, both natives of Florence.