Normally the extension should be requested in the same city from which you have received the letter of invitation. In our case, for example, the invitationcame from Yogyakarta. But we, at the time of the visa's expiry, were on the island of Lombok. Not easy to get back to Jogja from there!
So then, how then to extend the visa and not having to go back each month in Yogyakarta or wherever you got the invitation from?
There are two possibilities: the first is to contact an agency, just google and you will find a lot. We found an agency in Mataram. They asked 750.00 rupiah each, 52USD. Definitely too much for us, and this was the cheapest we could find. But if you have money and you do not want to go to the immigration office this is the hassle free solution.
The second solution, for those like us who do not have much money, or don't want to pay an exaggerated fee to the agency, is to do it yourself. The cost for the extension is 350,000 rupiah, 25USD, less than half.