Here's some useful info on charges for tourist visas and rules & regulations concerning them.
The Indonesian authorities are extremely strict on these issues and don't take them lightly. If you overstay your visa, it will cost you to the tune of around $20 US per day. On top of this you still have to pay the rather annoying 'Departure Tax', which itself is around $15 and, apparently, is only payable in Indonesian Rupiah. For which reason, keep a few bills for the airport on your way home!
