OTTAWA - Prime Minister Stephen Harper and his Conservatives are paying just a small fraction of the cost of partisan and personal junkets aboard the military's fleet of Challenger executive jets.

And documents show that the Prime Ministers' Office changed the formula for calculating flight costs after Harper's first partisan journey - a move that slashed subsequent Conservative party repayments.

Neither the original formula nor the reduced charges came anywhere close to what Harper himself in Opposition had called "$11,000 per hour Challenger jet flights" by the previous Liberal government.

The invoices, obtained by The Canadian Press under the Access to Information Act, show a total of three Challenger flights by Harper in 2006 for which the military billed the Prime Minister's Office.

The first flight was Feb. 10, 2006, shortly after the minority Conservatives won power. Harper's return trip to Halifax from Ottawa for the retirement party of Nova Scotia premier John Hamm was deemed a partisan exercise, and the Conservative party paid the freight.

The invoice from National Defence, which lists Harper and six staff on board, calculated the trip cost "3.1 flying hours X $2,139.00/hour."

The total bill: $6,630.90.

On July 26, Harper and a contingent of six staff flew from Ottawa to Moncton, N.B., for a Conservative party event. That time, however, the party reimbursed National Defence less than half the cost of the first flight for roughly the same flying time.

The price was dictated by the PMO in an e-mail to National Defence:

"Following up on our telephone conversation, it is the wish of the Prime Minister's Office that the Conservative Party of Canada compensates the Crown for the use of the Challenger on July 26," wrote a PMO official on Aug. 10.

"Our travel agent . . . has advised us what the airfare would have been had the prime minister and his staff flown on commercial scheduled flights to Moncton, N.B. Return airfare would have been $483.72."

The e-mail lists the staff onboard and calculates the total owing as $3,144.18.

Commercial airfares were also used to calculate Challenger costs when Harper, his son, Ben, and five staff flew to Toronto on Oct. 4 for a Toronto Maple Leafs game.

Sandra Buckler, Harper's director of communications, said the commercial rate was decided upon after discussions with various government departments and the private sector.

"There was no previous protocol, as the former Liberal government never reimbursed Canadian taxpayers when they used the Challenger for non-government business," she said in an e-mail.

"Aside from the first instance, Canada's new government has been consistent in its protocol for reimbursing the cost of an economy return trip ticket. We believe this is a fair balance, given the fact that the prime minister, for security purposes, must travel privately."

Liberals who bore the brunt of the former Tory mantra of $11,000-per-hour flights were not impressed.

"They're trying to say they're whiter than white," fumed Liberal MP Denis Coderre. "But it's worse, because they're playing with the numbers."

Curiously, a flight by Harper to Charlottetown, P.E.I., and Moncton on April 28-29 for a provincial Conservative party fundraiser was not reimbursed as a partisan excursion.

Buckler said that trip included a "roundtable meeting with Block Parents" and was therefore considered government business.

That kind of fluid partisan-government mix was a frequent target of Conservative complaints when Liberal prime minister Paul Martin was in power.

The Liberal party did not reimburse the federal treasury for any of the flights. But Martin personally did pay for his family's vacation to Morocco aboard the Challenger in December 2004, basing repayment on commercial airfare costs and then almost doubling the figure.

Prime ministers are required by security to fly only non-commercial, and filling the extra seats with staff makes economic sense.

However, the subject is sensitive for Conservatives because they made so much noise about Challenger flights during previous Liberal governments.

Harper himself and at least half a dozen members of his cabinet, not to mention numerous backbenchers, repeatedly hammered the Liberals for what they called "flying limousines" that they alleged cost taxpayers $11,000 an hour to operate.

"As they pad their expense accounts and look out the windows of their $11,000 per hour Challenger jet flights to B.C., they think everything is going pretty well," Harper said of the Grits in a speech in Vancouver on Oct. 12, 2005.

Harper had also accused Martin of flying "around the country on a government jet at taxpayers' expense" while dropping "enough money to cover up the stench of corruption."

Now, having been elected to office on a platform of transparency and accountability, the Conservatives are finding that accounting for Challenger flights is more challenging than they imagined.

Linking Challenger reimbursements to the actual cost of operating the jet is extremely difficult.

The military's 2005-06 Cost Factors Manual for aircraft, also obtained under the Access to Information Act, lists the full cost of operating the administrative Challenger used by the prime minister as $9,124 per hour.

A utility version of the same jet, used by the military, costs $11,541 an hour to operate.

Given that most of these are fixed costs that accrue whether the jet is flying or not, establishing a billable hourly flight rate is problematic.