A report by an accounting firm says the Harper government did not give enough leeway into the plan to buy 65 F-35 fighter jets.
The Harper government has said the purchase price would be nine billion dollars.
The K-P-M-G report says uncertainties in the program could make the air force cut the number of planes to 55 or force the Conservatives to up the purchase amount by between 1.5 and 2.5 billion dollars.
K-P-M-G estimates the cost as 45.8 billion dollars over a 42-year lifespan.
Defence Minister Peter McKay says the government is setting the re-set button and will get the best fighter for the Royal Air Force and the taxpayers of Canada.
