I can't speak for the geography, but people have already estimated the cost of the wall - legitimate sources, like CNN, who discussed the cost with engineers.

The wall would cost around:

$10,000,000,000

Obama just pushed a fiscal year budget of:

$4,000,000,000,000

$4,000,000,000,000 - $10,000,000,000 = $3,990,000,000,000.

Considering the wall would also take about 4 years to make, splitting that $10 billion across 4 years, its actually like this:

$4,000,000,000,000 - $2,500,000,000 = $3,997,500,000,000

Which makes the cost of building the wall 0.063% of the current fiscal year budget for the federal government over the period of 4 years.

Even with cost overruns, the cost would be statistically insignificant.