I hired a small Daewoo Matice for two days for 102 Euros. It was fine for a couple of days driving around the island (had air con too).
A freind hired a car over the internet for 10 days in UK prior to arriving in Ibiza for 18 pounds a day (he did some research and suggests its much cheaper if you book in UK rather than when you arrive in Ibiza).
Dont forget to take your driving licence with you.
Ibiza roads are not bad. But driving in Ibiza Town can get a bit hairy, v busy, lots of one-ways, roadworks and difficult to park.
Good Luck.