A steel wall separates Nogales, Sonora, Mexico from Nogales, Ariz.
A steel wall separates Nogales, Sonora, Mexico from Nogales, Ariz. - 
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SCOTT JAGOW: Today the President signs the Secure Fence Act of 2006. It calls for the government to build a 700-mile barrier along a stretch of the U.S.-Mexican border. What the bill doesn't provide is the money to pay for it. And oh, it will be expensive. Here's John Dimsdale.

JOHN DIMSDALE: The new law calls for building a double-layered wall with roads, lighting, camera and sensors along a third of the distance between the Pacific Ocean and the Gulf of Mexico.

The cost estimates range from $2.2 to $6 billion. Most of that will have to be appropriated in future budget bills.

STEVEN CAMAROTA: A fence is a good start.

Steven Camarota with the Center for Immigration Studies thinks it will be money well-spent.

CAMAROTA: It probably does have some deterrent effect, where large numbers of people come across and sometimes overwhelm border patrol. That becomes much more difficult when you put in a system of fencing.

Opponents range from environmentalists worried about migrating animals to a Native American tribe that straddles the border.

The Department of Homeland Security has argued that a virtual fence, using land-based and airborne sensors could also do the trick, at a lower cost.

In Washington, I'm John Dimsdale for Marketplace.