Chris
Hi Daniel, If there’s something in your building contract that gives you the understanding that it’s ‘fixed price’ then it would be my assumption that it’s not a completely standard RMB Residential Building Contract. Because I have a copy of the 2011 form of that contract, and it would appear to me that Clause 54 allows the builder to charge you for fluctuations in the cost of pretty much everything (subbies, materials and services), apart from possibly the cost of his own direct employees.
I’d suggest you read everything in your contract carefully, and if other clauses contradict this, or it’s not clear to you, then consult a contract lawyer.