According to the
holden website, a 90000km fixed price service for 2010 VE Berlina costs $672. Oddly they don't list a LPG variant or a 150,000km service but for a petrol vehicle i'd think its a repeat of 60,000km service which should cost $491. Whether your vehicle qualifies for the fixed price schedule i have no idea.
As to LPG servicing, it will obviously cost more than petrol only vehicle as a few more things need to be checked and possibly lpg filter replaced. Then there is the cost of meeting your legal obligation as the
tank approaches 10yo since it must be removed, inspected and tested before its restamped and put back into service before the 10 years are up. Obviously you want the tank empty for such tank testing work. Tank testing alone can be from $100 to $300 or more if removing the tank is a difficult job.
So it may be that the quoted $892 service doesn't sound unreasonable for Holden, especially if it includes tank testing.
As to who can do LPG service, only mechanic who are gas certified (AAFRB?) can work on LPG systems. This requirement also includes Holden dealers which must have an gas qualified mechanic to work on LPG systems. LPG tank testing may require another level of certification on top of that but i'm not sure.
As always, you don't have to go back to Holden for your service, any suitably qualified business will do. A quick google search found
this mob in Dandenong so check what they charge for LPG service. Keep in mind i don't know this mob from a bar of soap as google found them. There may be someone closer to you so try some google foo yourself
Good luck with it.