New Yorkers lucky enough to own a building with one of street artist Banksy's newest works have taken to hiring security guards. And with good reason -- similar works in England have sold for more than $1 million.

In the South Bronx, building owner David Damaghi hired two guards to protect a Banksy work depicting a small boy spray painting while a butler held his paint cans, the web site Gothamist reported. In the Red Hook section of Brooklyn, a building owner hired a guard after a different graffiti artist painted his own tag under a Banksy red balloon. And in Williamsburg, owners installed a pull-down metal gate and hired guards to protect a piece after it was almost defaced.

Banksy's outdoor paintings, cut from walls, doors and the sides of buildings have sold for more than $1 million. In June, a mural of a boy sewing a Union Jack flag, known as "Slave Labour," sold for $1.1 million at a private auction. The work had been removed from the wall of a local retail shop in North London in February.

Even without an auction, having Banksy's work visible on a building can lift its value. After Banksy painted a huge rat on the side of boarded-up Liverpool pub in 2008, the building's value increased by an estimated $800,000.

Street art by Banksy has sold for between $350,000 to more than $1.2 million, says Robin Barton, who runs the Bankrobber Gallery in London and deals in such works. But calculating the value of the recent works may be tricky, as they're less impressive to Barton. "I don't particularly rate the new NY works," he says.

Sales of Banksy's outdoor art have been controversial. While a group known as Pest Control validates the authenticity of Banksy's gallery art, it does not opine on street works. Major auction houses like Christie's won't sell alleged Banksy works lacking the validation.

Banksy sanctioned a small sale of signed canvases in New York, but with his usual sense of humor. At a stall in Central Park, a man working for Banksy sold the artist's canvases for $60, a fraction of their market value. Banksy later posted a video of the stall, including a tourist haggling down the price in one sale.

The mystery British graffiti artist has been in New York City all month, working on what he calls his "Better Out Than In" visit. No one has disclosed Banksy's identity since he first began spray-painting his iconic images in 1993 in England.

His works in New York have ranged from a depiction of the attacked World Trade Center towers with a flower emerging instead of a plume of smoke, to a silhouette of a dog relieving itself on a fire hydrant. Banksy has been posting pictures of each new work in New York City on his web site.