Donald Trump has been under fire since he called Mexican immigrants drug traffickers and rapists during his presidential election announcement speech on June 16. He has also suggested that the U.S. should do all it can to stop illegal immigration, including building a wall on the southern border and deporting the nearly 12 million undocumented workers who are living in the country illegally.

Although Trump's controversial policy on immigration has ironically helped him take lead as the GOP front-runner in the 2016 race, experts say it would actually hurt America in the long run if implemented.

According to a 2009 report issued by the Government Accountability Office, fencing costs $2.8 million to $3.9 million per mile on average to install. Taking that cost as a general formula for a complete project, constructing a wall on the entire southern border would cost around $20 billion. Plus, that hefty price tag does not include maintenance or staffing.

"You don't necessarily get a lot of bang for your buck," Tom Wong, an assistant professor of Political Science at University of California, San Diego, told CNN.

Meanwhile, Marc Rosenblum, the deputy director for the U.S. Immigration Policy Program at the Migration Policy Institute, said that "costs go up quite dramatically" in more remote and desert areas. As a result, a wall would cost about $58 million for 3.5 miles in difficult terrain within the San Diego section alone.

"I think that it's possible, but it would prove to be extraordinarily expensive," Rosenblum said.

Even if the U.S. was able to sponsor the construction of a wall, Rosenblum pointed out that Trump would likely take issue with the project due to private land ownership. A large part of the land on the border is either privately owned or considered as tribal territory, he said.

Plus, Rosenblum noted that a wall still could not stop illegal migration completely.

"We know immigrants can get over and under walls," he said.