"I think everyone would accept a 60 per cent increase in funding over a five-year period when the number of disposed cases haven't increased anything like that would suggest there are some inefficiencies there."
This year's funding has been capped at $43.2 million - down from spending of $51.8 million in the last financial year. Each firm's cap is based on its previous fees and expected casework - and on average they will get 9 per cent less.
Mr Hampton said the Crown Law Office was also trying to find out why some Crown solicitors' average costs were much higher than those of others for handling similar cases.
"There is considerable variation between Crown solicitors on their average costs. Some of them dispose of cases for half the price of others."
He said the Crown Law Office was working with the solicitors to find out what drove the cost differences and would then set up a more long-term funding model. The cap for each law firm was an interim measure until that model was developed.
Previously the Crown prosecutors have billed the Crown for their services, based on hourly rates. However, a cap has been set for each firm this year while the Crown Law Office works out an alternative funding system to try to address the spiralling costs.
Attorney-General Chris Finlayson said a review of the Crown Solicitors' Network by John Spencer had found the work was of high quality "but the cost of the network has increased considerably in recent years".
Mr Finlayson said the aim was to return spending on Crown prosecutions to 2009 levels, rather than continue to top it up every year.
He said it was a significant change to make in one year, despite the drop in cases, so there was one-off extra funding of $4 million for the first year of the cap.
Labour MP Charles Chauvel said that even if there was a drop in the number of cases, many of the law firms now faced more complex legal work, which added to their workload.
Mr Finlayson agreed the work was more complex, partly because of reforms to the criminal justice system under National.
But he could not keep going to the Minister of Finance for extra funding and a "more rigorous approach" was needed.