Otherwise it would cost Canada $61 billion to be part of the system, said Trump, who has repeatedly called for the United States' northern neighbor to become the 51st state.
Canada has expressed interest in joining the missile system -- plans for which Trump unveiled last week to defend against a wide array of enemy weapons -- but has firmly rejected any loss of sovereignty.
"I told Canada, which very much wants to be part of our fabulous Golden Dome System, that it will cost $61 Billion Dollars if they remain a separate, but unequal, Nation," Trump posted on his Truth Social network.
"But (it) will cost ZERO DOLLARS if they become our cherished 51st State. They are considering the offer!"
There was no immediate response from Canada to Trump's claims.
Trump announced plans for the "Golden Dome" system a week ago, saying it would eventually cost around $175 billion and would be operational by the end of his term in 2029.
Experts say the scheme faces huge technical and political challenges, and could cost far more than he has estimated to achieve its goals.
Trump also said at the time that Canada was interested in joining the missile system. Canadian Prime Minister Mark Carney then confirmed that his country had held "high level" talks on the issue.
NATO members Canada and the United States are partners in continental defense through the North American Aerospace Defense Command (NORAD).
But the scheme now seems set to add to the tensions that Trump has sparked with Canada.
Carney politely but firmly dismissed Trump's calls for Canada to become part of the United States when he visited the White House earlier this month, saying his country was "never for sale."
The Canadian premier and Trump did however appear to smooth over some of the strains over the tariffs that the US president has slapped on Ottawa.
Related Links
Learn about missile defense at SpaceWar.com
Learn about nuclear weapons doctrine and defense at SpaceWar.com
All about missiles at SpaceWar.com
Learn about the Superpowers of the 21st Century at SpaceWar.com
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