OTTAWA - Taxpayers will shell out almost $4 million to fill seven seats in the House of Commons left vacant after MPs retired less than 18 months after winning election.

It costs an average of $565,000 to stage a federal byelection, according to Elections Canada.

The hefty price tag has become an issue as Tories and Liberals trade taunts over the timing of the byelections.

Five of the seven vacancies were created by Liberal MPs opting to seek other job opportunities. And three of those Liberal departures were orchestrated to create openings for high-profile newcomers, including former leadership contenders Bob Rae and Martha Hall Findlay.

"The Liberals will have to justify the costs to voters in each riding,'' said Conservative party spokesman Ryan Sparrow.

He added that the departure of so many Liberal MPs reflects Stephane Dion's "weak leadership.''

But Liberals counter that Prime Minister Stephen Harper will add unnecessarily to the expense if, as Tory insiders say is likely, he chooses not to call all the byelections at the same time.

Elections Canada spokesman John Enright said there would be some central administrative savings if all byelections were held simultaneously but the savings would be "minimal.''

More importantly, Liberal national campaign chairman Mark Marissen said Harper shouldn't leave some ridings unrepresented for any longer than necessary.

"We know that they're open. Why wouldn't he do them all at once?''

Harper must name a date for a byelection in at least one riding -- Montreal's Outremont -- by July 28 and Tory insiders expect he'll call it, and two other Quebec byelections, for early September.

He'll likely delay byelections in the four other ridings -- two in Toronto, one in northern Saskatchewan and one in Vancouver -- until November.

Liberal Leader Stephane Dion told The Canadian Press he'd prefer all seven to be held simultaneously. He said there's no justification for delaying some byelections other than "very narrow partisan considerations.''

Dion declined to specify what those considerations might be. But Marissen said the Tories are "afraid'' the Liberals will sweep five of seven ridings, enabling them to show the party is "alive and well.'' Spreading the byelections out over a few months would dull the sense of Liberal momentum.

Marissen said the Tories are also afraid of facing Rae and Hall Findlay in the Commons and are, thus, looking to delay the two Toronto byelections where the pair are set to run.

Hall Findlay echoed that assessment.

"If Stephen Harper decides for whatever reason to delay . . . I'd have to question is he afraid of a girl?'' she taunted.

For his part, Rae said: "I don't see any reason for a delay.''

But Dimitris Soudas, spokesman for the prime minister, said it's Dion who's wary of having two more former leadership rivals in the Commons.

"I think Mr. Dion is more afraid to get Mr. Rae and Ms. Hall Findlay into the House than anybody. He's already got Mr. (Michael) Ignatieff overshadowing him.''

Soudas stressed that the prime minister has six months after an MP resigns to call a byelection and noted that previous Liberal prime ministers have often waited the full six months before naming a date.

Of the current vacancies, the six-month period is almost up for Outremont and Saint-Hyacinthe-Bagot. One other Quebec riding, Roberval, is technically not vacant until the end of the month, although Bloc MP Michel Gauthier gave notice of his retirement months ago.

The four other ridings have all been vacant for less than a month.