Along with doing without all the headaches involved in driving your own vehicle, there are other distinct advantages to hiring a driver. Comparison shop to find a driver who can also serve as guide, i.e., provide some cultural/historic context to what you are viewing. A good driver will also know ot all the secret shortcuts in the back alleys of Kuta and Legian.
In some cases, depending upon where you are or who you rent the vehicle from, it can cost just as much to hire a vehicle with driver as it costs to hire a vehicle without. This might be the case, for instance, in hiring a vehicle from your homestay owner who may have his eldest son do the driving just to keep you safe and protect his investment.
As a rule, however, count on it costing as much as Rp 75,000-80,000 per day (10 hours) extra to have the services of a driver. For overnight tours, it will cost you total of Rp200,000-250,000 because the custom is that you pay for the driver's meals and accommodations. Or give him this as part of his per diem and let him cover his expenses himself. Hotels in Bali often have quarters and special meals set aside for drivers.
One expense you will not have is the daily cost of insurance as this is usually assumed by the rental agency when they rent out a car with driver. In the very least, you are would be required to carry less insurance than it you drive it yourself. Another advantage is that a driver will be responsible it anything mechanical goes wrong or it there is a traffic violation or accident. Also a driver can guard your belongings inside the vehicle while you are not there. In the Kintamani area, the locals may even disconnect your fuel pump in order to convince you that you need mechanic.
A word of caution. A driver will probably get commission for delivering you to a posh restaurant, gallery, or art shop. Do not give in to all his suggestions, especially commercial ones. He also may think you are not able to eat anywhere but in a fancy overlook restaurant where you may end up paying an enormous amount (Rp 85,000 and up per person). Suggest instead that you eat lunch in a local warung.