They are actually M2A2 howitzers, which are 105mm guns, not 25 pounders. I think the majority of these guns in Aust service were upgraded to M101 spec in the 60's and 70's, so very few will still be in original M2A2 configuration.
The $4000 mentioned is not the price of each gun, it is only for the De-Mil work required before the Army will release them.
See
Minister's Press Release.