The parliamentary budget officer is estimating that changing the rules for conditional sentences could cost taxpayers an extra $145 million because of increased parole reviews and prosecutions resulting in trials.

Changing the eligibility rules for conditional sentences is one of the proposals in the government's omnibus crime bill, which is now in the hands of the Senate. A conditional sentence, sometimes called "house arrest," allows for offenders to serve their sentences in the community.

Kevin Page, the federal budget watchdog, issued a report Tuesday in response to a request from NDP MP Joe Comartin to examine the potential costs of changing conditional sentences.

His analysis determined that had the eligibility criteria for conditional sentences proposed in Bill C-10 been in place in 2008-09, the federal government would have been paying an additional $8 million that year and provincial governments would have been on the hook for an extra $137 million.

The increased costs would have been due to more parole reviews and trials and housing of prisoners. Page says the figures are likely underestimates because they don't include additional capital costs related to building new prisons if necessary.

His report shows that had the proposed rules been passed in 2008-09, an extra 4,468 people would not have received house arrest.

He also said the average cost per offender will rise from approximately $2,600 to $41,000 as a consequence of the changes.

Page assumes that if conditional sentences weren't available for certain offences, people accused of those crimes are less likely to plead guilty and will take their chances on a trial. That will drive up costs for the court system, the report predicts, and for the corrections system because of those who are found guilty and sent to jail.

New inmates will eventually be eligible for parole, and there are added costs for that system, Page said. A single review by the Parole Board of Canada costs an estimated $4,289 according to his calculations. Page said Public Safety Canada was asked directly for an estimate and it did not provide one, so he relied on other figures.

The provinces have their own parole boards to review applications from offenders in their custody and those costs range depending on the region.

PBO had trouble getting government data

Figures in his analysis are based on a number of assumptions, including that 50 per cent of offenders who would have previously taken a plea bargain and pleaded guilty would instead opt for a trial if conditional sentences aren't an option.

The budget officer said he had trouble getting data from Public Safety Canada and from the Public Prosecution Service of Canada and that he had to rely on Statistics Canada reports and Department of Justice reports.

The budget officer said in his report that his findings contradict information provided by the government to the House of Commons in October 2011. According to Page, the Conservatives said changes to conditional sentencing would come with "no federal costs."

The government has said that in total, Bill C-10 is expected to cost $78.6 million over a five-year period and that a number of its measures will mean no added costs. That figure doesn't include costs for the provinces and some provincial governments are estimating their expenses could reach as high as $1 billion.

Page only looked at the conditional sentences element of Bill C-10 in Tuesday's report. The new mandatory minimum sentences proposed in the bill are also expected to be a driver behind increased prisoner populations and therefore costs. T

The NDP asked Page to look into those estimated costs in addition to the conditional sentences, but MP Jack Harris said Page advised the party the methodology for that would be extremely complicated. For now, he's only done the analysis of the house arrest costs.

The government consistently defends its crime agenda by saying its costs are justified and the Conservatives were given a mandate in the last election to move forward with it.

Julie DiMambro, a spokeswoman for Justice Minister Rob Nicholson said the government is committed to toughening the sentencing for serious offences.

"Conditional sentences, or house arrest, should not be available for serious crimes such as sexual assault, kidnapping, and human trafficking," she said in an email. "That is why the government has proposed this legislation — to ensure that individuals who commit serious and violent crimes serve prison sentences that reflect the severity of their crimes."

She also noted the federal government transfers funds to the provinces for its programs.

The NDP reacted to the report by criticizing the government over its assertion that restricting the availability of conditional sentences wouldn't cost anything.

"They never bothered to figure of the consequences of their legislation or the cost of the legislation," Harris said. "It shows that this bill is really irresponsible in many ways but is clearly ideologically driven and driven for the purposes of [public relations]."