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Photographer: PAUL J. RICHARDS/AFP/Getty Images Photographer: PAUL J. RICHARDS/AFP/Getty Images

Donald Trump said he wants to deport everyone who's in the U.S. illegally, but he doesn't want to talk about how much it would cost.

One study by the American Action Forum, a conservative pro-immigration group, estimated mass deportation to cost from $400 billion to $600 billion over a decade, and reduce economic growth by $1.6 trillion.

"Well, first of all, they're wrong," Trump, the Republican presidential front-runner, said Sunday on ABC's This Week referring to the estimate.

He argued instead that undocumented immigrants are costing the country and he would rely on "good management" to deport the estimated 11.3 million people in the country illegally.

"You have so many illegals," he said. "We don't even know how many illegals. I hear 11 million. I hear 30 million. The government has no idea. We have lost control of our country. We've lost control of our borders."

Trump attacked rival Jeb Bush, who supports legal status for undocumented immigrants and said Bush could never carry out mass deportation.

"Jeb is a very low energy person," he said. "He'd never be able to do it. He's the one that said they come out of an act of love."

Trump also mocked Wisconsin Governor Scott Walker for walking back his remarks after earlier this week signaling support for ending birthright citizenship, as the New York businessman has proposed.

"Scott Walker has changed his mind now," he said, "because he keeps going back to his pollster, and his pollster says, 'oh, Trump has a good idea, oh, Trump has a bad idea, oh, no, wait a minute, Trump has a good idea.'"

Appearing on ABC after Trump, Walker said he doesn't support repealing or altering birthright citizenship under the Constitution.

"My point is any discussion that goes beyond securing the border and enforcing the laws are things that should be a red flag to voters out there," Walker said.