Maripaz,
can you confirm that you are extending a Visa on Arrival (VOA) rather than a social visit visa (Sosbud) as there are important differences, most notably the need for a local sponsor for the Sosbud.
The basic cost of the extension in either case is 500k Rp payable at a bank as immigration no longer handles payments. I recently spoke with someone who paid 900k Rp inc. immigration fee to use an agent to extend a VOA. Expect to find wildly different quotes and expect to pay more for a Sosbud if the agent is also acting as the sponsor perhaps.
Immigration is in Jimbaran so the choice does depend on where you are located here and how mobile you are. If you are comfortable riding a scooter around, and say below the line running from Canggu to North Sanur I'd agree with Mark, do-it yourself so you can see and understand the process. If you arrive at the office prepared at 8 - 8.30 you will be out by 9.30 and can then use it as an excuse to explore Jimbaran or Ulluwatu for the day. Or if you live in the deep South an excuse to head North. If you need to make three return taxi trips of any distance the agent cost is going to start to look cheap.
You will need to bring along your original passport, a photocopy of the photo page, a copy of the visa stamp page and a copy of your onward travel ticket out of Indonesia to get a VOA extension. For a Sosbud you dont need the outgoing ticket but you need copies of your sponsors KTP and a pro-forma sponsor letter from your sponsor to apply. If not using an agent your sponsor must be physically present for the first visit. Theres a copy shop run out of a truck parked in the front yard normally but I prefer to be ready when I arrive copies in hand.
At reception they'll check your docs are present and correct and give you a form to complete. You are supposed to start the process a week early though staff are generally very reasonable if you come late but its no longer possible once you visa has expired on the 30th day with your arrival day counting as day 1 - Don't get caught out!
As noted in an earlier reply, you will need to make 3 journeys in total rather than 1 so this may be the decider for you.
Hope this is helpful.
Regards