@Gormon.
Cite facts. The amount to keep someone in prison ranges from $20k to $80k depending on the level of security. A quick google search and the average of the results of the first few pages supports this. Commit murder in Australia and you're looking at a minimum 12 years in jail. $240k - $960k. In murder cases, the brunt of the cost is in the hands of the defense, not Regina, and would never cost more than a million dollars.
In Texas, a death penalty case costs an average of $2.3 million, about three times the cost of imprisoning someone in a single cell at the highest security level for 40 years.