The price for the volcano climb was 250.000rp each. This was a based on the fact that there were 6 in our group. You can get a price close to that easliy though I'm sure or just pull together a group while you're over there. You'd be surprised how easy it is to get people on board over a casual poolside chat. Most people would like to do stuff over there but don't know how to get it organised at the right price.
Alternatively they may have a group ready to go when you're ready.
Happy Travels