The idea used by Mr Chauvel in this media report seems to be that the relevant costs will decrease as more work is done. This supposes that the relevant calculation is costs per case.
I think that the better calculation is costs per funded hour in court by counsel appearing in a case. So, for a year, take the total costs that are publically funded for the practice, and divide it by the total publically funded hours spent by counsel from that office in court. For the private bar, average those costs per hour to get costs per hour per firm, and then compare that average to the costs per hour for the PDS office.
This way of looking at it asks how much does it cost to put a PDS lawyer in court compared to putting a publicly-funded private lawyer in court. The "apples" are court-hours. This is a neutral basis for comparison. If the "apples" were cases the comparison would be complicated by the various kinds of cases and whether the best legally available outcome for the client was achieved in each.