Anyone who can afford it, which is just about every expat employed by a non-Indonesian company hires a driver. Anything else is overtime. Most drivers WANT overtime. The average salary is about Rp 400,00 per month. Some individuals pay less but usually not less than Rp 300,00.
Some companies pay their drivers more.
Some families have two vehicles and two drivers. The city is very spread out and taking public transportation is really not a choice too many people take. Taxis are plentiful but if you don't know where you are going the taxi driver probably doesn't know either. Traffic really can be a nightmare. Maps are available but not necessarily accurate.