Some quick calculations just for fun. Judging by the scale of the car, these steel bars are 8 m tall above the ground, 0.25 m in diameter and spaced 0.25 m apart. Assuming they are anchored 2 m into the ground and made from solid steel, they would require about 40 million metric tonnes of steel - almost half USA's total steel production in 2014 - to cover the entire US-Mexican border with this design. Assuming a price of 335 USD per tonne (the current U.S price for shredded scrap steel), the cost of the raw materials alone would be more than 13.5 billion dollars.
That's for the bars alone, not including the necessary bracing and such.
Of course, they could be made from hollow steel tubes but this design is critically flimsy even when solid so that's not really an option. All it takes to break through it is a truck, a length of heavy steel chain and about half an hour of time before the border patrol arrives. An actual wall that would have any significant effect at all, would require far sturdier bars using far more steel.
On top of that, steel rusts, so maintenance would be a nightmare.
Please feel free to repost and redistribute this post any way you like as long as it is the entire post (excluding the illustration from Twitter) with the reservation that this is a very quick estimate and should be checked and rechecked for errors by experts.