Most agencies allow you to book a car online in advance and at a cheaper rate. Prices are typically cheapest between September to October during the low season, although it isn’t uncommon for agencies to overbook, meaning your pre-booked car may not be available upon your arrival.
A standard rental car will cost you somewhere between CUC45 (£35) and CUC85 (£70) per day, with unlimited mileage. Insurance is normally paid additionally, often in cash, and charged at CUC10-20 (£8-15) per day.
Cuba is atrociously signposted and GPS devices are forbidden, so a good map is essential. Car hire companies will provide one, but consider purchasing the superior International Travel Maps Cuba, or the hard-to-find Guía de Carreteras in advance.