Thanks Richard,  your explaination is clear for me and no doubt that they don't show me details of calculation method. I understand that it's complex pretty much. However I'm wondering if something is unreasonable, I narrow down my case as below few lines:

1) 15 Oct. variable rate: 7.88%, break cost: $10000  (got reply within 3 hrs)
2) 10 Dec. variable rate: 7.13%, break cost: $17000 (got reply in a week)
3) 15 Dec. variable rate: 6.13%, break cost: $18000 (got reply within 3 hrs)
4) 22 Dec. variable rate: 6.13%, break cost: $19000 (go reply in within1 hrs)

I raised my request on 8 Dec to query the break cost but got reply on 15 Dec, when they issued new rate 6.13%. I argued them why delayed my request a week? They said sorry to me and also told me no big difference between two weeks.  I dont understand why 1% reduction of variable rates had no big impact on the break cost.

I was going to query break cost on 8 Dec and get reply on same day and then made a decision if I would break my fixed rates because I have predicted the bank must decrease the rates in one or two weeks. Almost everytime, it cuts the rates 2 or 3 weeks later after CBA published new rates. However I got reply late for a week especially I called them and physically got in their office to follow up totally more than 4 times. It's unbelievable and unacceptable. It's another story but it's why I suspected if such break costs are reasonable.

Thanks once again for your explaination and hope you all have a merry Christmas and a happy new year.

Tom