andrewNZ: I'd say a large part of the price difference is down to the building company buying loads of them and therefore getting them at a very good price. 8k is a big difference though
We get iron both longrun and tiles at a very good rate and even the rrp of $15 l/m for coro vs our cost price for tiles the difference is no where near 8k. There is seriously something fishy about the 8k price difference.
I would ask the builder for a breakdown of costs in regards to the roofing. If you want I would be happy to have a look at the plans and give you a ballpark quote so you have a more accurate idea of cost if that helps at all.