I mean, I feel like that was an ignorant comment and I'm not entirely sure what that would accomplish... But I'm here to do math. The Sahara is 2,983 miles ^source at its longest point whereas the US-Mexico border is 1,954 miles long.^source. So honestly, it's not off by that much. When you consider that Morocco and Albergia are your main two countries you'd need to truly lock down, the hypothetical wall gets a lot smaller.

Building a Saharan wall would probably be cheaper than a US-Mexico because of (lack of) wages paid. Cost estimates with President Trump's proposed wall have been all over the place, but I'll take the Department of Homeland Security February 2017 estimate of 21.7 billion to build an additional 1,000 miles of wall ^source. A line across the Sahara based on U.S. estimated cost would cost 60.9 billion to build.

Only one of the three articles I looked at had the quote, which I guess he actually said

"You need to build a wall around the Sahara,"

which is a lot more than 3,000 miles. I couldn't find and solid circumference numbers but I imagine 8,200 miles would get us in the ballpark (2x length + 2x width). Now we're looking at a cost of 193 billion, or about 15% of Spain's GDP ^source... to build a wall across a desert they don't share a land border with.

Anyways, I should get back to work.