Last updated by Ryan Whitacker on March 17, 2017.

Americans: prepare to get out your wallets to pay Uncle Sam. After sifting through Trump's budget proposal, Reuters recently reported Trump “wants Congress to shell out $1.5 billion for the border wall with Mexico in the current fiscal year – enough for pilot projects to determine the best way to build it – and a further $2.6 billion in fiscal 2018” citing White House budget director Mick Mulvaney.

Some were surprised Americans would have to pay for the wall. To those in touch with reality, though, it seemed obvious from the start that Mexico would not be paying for the wall. Mexico's president rightly Tweeted, “… Be clear with US tax payers. They will pay for it.” By the way, does anyone else remember Trump saying Mexico would also pay for the cost of deporting undocumented immigrants? In case anyone's wondering, that hasn't happened and is unlikely to ever come to pass.

What surprised us was not that Americans would pay for American activity, but the shockingly low figure of $4.1 billion over two years to pay for one of the most ambitious walls ever built. Not only is that number far removed from Trump's prior quote of $10-15 billion, but it's even further removed from reality.

The Big Wall: Actual Costs

What's funny about the $4.1 billion cost is that it wouldn't even cover a fence. The US currently has a border fence that so far hasn't posed any real barrier to immigration. Not only is the fence incomplete, but it's not too hard to dig under, cut through, or go over. This meager defense has cost about $5.3 million per mile. There are 1,954 miles of border between the US and Mexico. A full-border pedestrian fence alone would cost $10.4 billion, or about 250% more than Trump wants to allocate.

What about a wall? There aren't may full walls to compare this to in recent history, but the best example is probably the Israeli West Bank Barrier.

So far this wall, which none would call a “beautiful” wall, has cost $12 million per mile (274 miles at $3.4 billion). While the ground in Israel isn't easy to move, it is relatively flat. It's easier to build a wall on the West Bank than than through the rocky, remote, and uneven land of the Sierra Madres. Construction may be even harder on the settling-prone sands of the Rio Grande winding through the Chihuahua Desert, the Sonora desert on the west, or the marshes and swamps on the east where the Rio Grande meets the coast.

It's hard to estimate the cost of such a huge project before doing any soil analysis or logistics estimates, but a very conservative estimate of a similar wall on this terrain is $15 million per mile. This is close to the CNBC estimate of $16 million per mile, and even conservative by White House Budget Director Mick Mulvaney's estimate of $8 to $25 million per mile.

But if we're conservative (which is rare for a US government project) at $15 million per mile over 1,954 miles the cost for the full border wall would be $29.3 billion. Taking the White House “worst case” figure it could cost as much as $49 billion.

The Most Over-Budget (or Under-Funded) Government Contract in History

The current benchmark for “cost overrun disaster” is the Big Dig in Massachusetts, where costs were $15 billion over budget. Actual wall spending would cost between $19 (@$12M per mile) to $45 billion (@$25M per mile) more than is budgeted. Most government projects turn out 50% or more over the estimate, so if the budget were $29.3 billion it wouldn't be surprising to see the cost reach $40 billion. Again, if we take the most generous numbers for Trump that's 7 times the number set aside in an already-controversial budget.

So What Now?

It's possible that Trump and company know $4.1 billion is not nearly enough to complete the wall. Perhaps the plan is to get the government (and US tax payers) committed to starting the wall. This was, after all, the way the wall was originally sold to Americans. First, get Americans to buy into the wall with a $0 price tag, then tell Americans they'll have to pay $4.1 billion, then slowly tell Americans it needs another billion here and there until you get the full $30 billion years later. Or perhaps there will be a small 100-mile section of wall built with fences throughout the rest of the desert.

It's unclear whether this is intentional deceit or simple naïveté, but this is certain: there is absolutely no way the wall is going up for $4.1 billion. Trump can (and likely will) argue this point all day, but the data is very clear.