President-elect Donald Trump says that his proposed wall between Mexico and much of the southern U.S. border will cost $8 billion to $12 billion, which is a very broad range. Recently, he said the United States would underwrite the cost of the wall and recoup it from Mexico.

Most experts believe the wall will cost well above Trump’s estimate. Trump argues as a builder and real estate developer he should know the number. There is no evidence he has sent an army of engineers and builders to either measure the likely length of the wall or estimate the materials that would need to be used or the costs of labor. That makes his estimate no better than a guess.

A recent analysis in the MIT Technology Review put the cost of the wall at as much as $39 billion. Its measurements are based on a wall that is only 1,000 miles long, with a height of 50 feet and a foundation that would go 10 feet below ground level. The estimates take into account concrete and steel costs, along with cost of labor.

The MIT Technology Review work ignores the proposed length of the Trump wall, so, despite its completeness, it has to be inaccurate.

Based on a Government Accounting Office estimate, calculated with the use of past construction of border walls, the average cost would be $2.8 million a mile. With that as a start, CNBC put the price at between $15 billion and $25 billion. However, the cost is based on a 1,300-mile wall.

Experts point out the several estimates do not take into account that the Trump wall would only cover 1,000 miles because rivers and mountains act as natural deterrents to those who want to cross into the United States from Mexico. At that length, a Washington Post came up with an estimate of $25 billion.

How much would the Trump wall cost? Even with detailed analysis and estimates, the final number will depend on the fact that how much money will be involved is like estimates for Navy ships, war planes and Air Force One. Trump’s number is almost certainly low, but it is impossible to prove otherwise, for now.

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