I would recomment you find yourself an agent and let him extend it for you. Normally you need to go to kantor immigrasie and need lots of letters, stamps and patience..
The prices for an extension do vary from 150.000 to 1 jt. per month. Somewhere in between should be fine, I guess. If you want to extend it again after that, you need to show up at the kantor immigrasie for a picture and fingerprint. (or at least... I had to)
But maybe anyone on this forum, living in Manado, knows the local customs of the kantor immigrasie.(and a number of an agent) You need to start the extension procedure about 1 1/2 weeks before your current visa expires.