DONALD TRUMP is a man of ideas. Although critics have lambasted him for flip-flopping on some policies (he now proposes to ban immigrants from "terrorist nations" rather than all Muslims), Mr Trump has stood firm on at least one proposal: his wall. A new report from Bernstein Research looks at the economics of the wall's construction.

The border between the United States and Mexico stretches 1,989 miles (3,200km), but the wall itself needn’t be as long thanks to the preponderance of natural borders such as the Rio Grande. Assuming a length of 1,000 miles and a height of 40 feet (12 metres), Bernstein reckon that the wall would require $711m worth of concrete and $240m worth of cement. Including labour, the total cost of between $15 billion and $25 billion is a bit more than Mr Trump's suggested $10 billion. (Bernstein’s estimates presumably do not factor in Mr Trump’s construction expertise.)

As it is not economically feasible to transport cement and concrete across great distances, the biggest business beneficiaries will probably be within 200 miles of the border. America has many more factories and quarries than Mexico, yet Mr Trump is adamant that the wall will be built with Mexican money. Cemex, a Mexican firm with around half the quarries close to the border, is likely to profit. At least some will benefit from the wall’s construction.