Opposition Leader Tim Hudak says $155 million is budgeted to renovate Ivor Wynne Stadium in Hamilton, but he believes it could be up to 40 per cent higher.
Ontario is contributing $22 million towards the stadium, on top of the other costs of hosting the Games.
Hudak says the city has an exclusive contract with one carpenters' union, so all construction projects must go through it.
He says closed tendering is an outdated practice that drives up costs, and will inflate the $1.4-billion price tag for the Games.
Charles Sousa, the minister responsible for the Games, says there's appropriate oversight in place to ensure the budget is managed well.
"Let's be clear, we make no apology for bringing the Games to Ontario," he said Monday in a statement that was released before Hudak's news conference.
But the Tories say the budget doesn't include some large costs, such as building the athletes' village — which falls to the province — or providing security for the Games.
"The budget itself that they've released is actually woefully inadequate," said Conservative Rod Jackson. "I mean, my high school prom had a more detailed budget than the Pan Am budget has been showing."
The federal and Ontario governments are contributing $500 million each for the Games. Ontario is also kicking in an additional $22 million to renovate Ivor Wynne Stadium.
The fiscal plan for the Games includes a $82 million contingency fund for cost overruns.