There is a very good costing of capability /costing of the various types on the Fire aviation site.
What is interesting is the difference between the 146 and the later RJ and the B737 v MD80 series.
Clearly at this stage the much lower purchase price is a major factor with it seems similar running costs.
The argument about filling times reflects the lack of permanent and temporary infrastructure. In North Amercia there are permanent bases with holding tanks which allow rapid fill and turnaround times. They also use air transport capable bladders for the same end. Its amazing how much water can be moved quickly with a breath of compressed air and a large hose?
It seems to me that this operation in Australian must be Federal and it must be standardised on 2 LFA types. That appears to be the B737 and the Bae 146 or RJ.
I wonder if the low LFA utilisation is a product of costs rather than ability to be far more productive?
Wunwing