A private car with a driver in Tunisia is priced per vehicle, not per person — so the cost is the same whether one person travels or five, and the per-head figure falls as the group grows. Day tours start at €74 for the whole car. There are 1,392 fixed transfer fares published on this site, so the price of a given route can be checked before anyone is contacted.
From about EUR 74 for a full day with the vehicle, depending on the route and how far it goes. That is the price for the car, so a family of four pays the same as a solo traveller.
No, and it is worth being clear about it. The driver knows the roads and the sites and will tell you about them, but a licensed guide is a separate qualification in Tunisia. One can be arranged at any site if you want the full commentary.
No deposit is taken. The fare is agreed before departure and paid to the driver, and cancellation is free up to 24 hours before.
Distances are straight-line, measured between the coordinates on our map of the sites; road distances are longer. Prices are per vehicle and come from the same database the 28 tours and 1,392 transfer fares are quoted from.