cost would vary depending on who you go to.
you have to replace the airbags you cant fix them once they are deployed.
you might need to replace parts of the dash depending on what is broken.
airbags might fall under the same category as seatbelts on the secondhand parts market so wreckers might not sell them so you might need to get them new from holden.
you might also need to replace the sensors that trigger the airbags.
you will also need a tech2 to diagnose the airbag system and also clear airbag errors as I don't think the cheap ebay obd2 devices can clear airbag deployment errors.
it is REALLY something you should get fixed by a professional with the correct tools and knowledge because if you do it yourself and your airbag deploys while you are driving down the road you could cause more damage and cost than you are trying to save doing it yourself.
I would ring holden for a quote to start with.