I have no claim on any insider knowledge of robust cost-benefit analysis data for 'The Wall', but I really wonder how many voters who think the wall is a great idea have actually spent any time in the open-desert borderlands? Not driven through a desert border crossing, but actually spent quality time working out in the open desert, in the mid-summer heat, dozens if not hundreds miles from anything but a scattering of human habitations? I have.
Personally, I've worked quite a bit in the desert near the border. Not on roads, not on established hiking trails, but out in the glorious expanse. I've actually met illegal groups trekking through on several occasions (including when I was working alone).
I have a close relative who was a border-patrol agent who actually patrolled the desert and busted illegals and incoming drugs (that person thinks the wall idea is absolutely moronic and a total waste of money and effort, even given the alternate technology that he had available to him back in the 1990s).
My DH currently works frequently in the border country in the open desert (different desert, different state), so I've gotten to see that area, as well.
The wall idea just seems ridiculous to me. Not only would you have to take away a bunch of ranchers' private property rights to build one, you'd have to disrupt and deface multiple national parks/monuments, disrupt local border town economies, put additional human pressure on fragile desert ecosystems, and (most importantly from my perspective) fuck up habitat, movements, and population gene-flow of vertebrate animals in the region. The ecosystem effects alone make such an idea a complete non-starter to me.
Then there's the fact that such a wall, once built, would require constant monitoring, manning, and upkeep, and I'm not convinced it would greatly reduce immigration. Hell, people still managed to get across the Berlin wall, despite it being 1) quite tall; 2) in a very restricted area that was heavily urbanized and thus easy to access, man, and repair; and 3) fortified with ~300 guard-towers with ARMED GUARDS WHO WOULD SHOOT TO KILL.
Also per my example of the Berlin wall, people understand how to tunnel under, breach, or climb over such walls (and have since medieval times). The Border Patrol already has lots of remote technologies in use to monitor the border. Back in the 1990s they already had advanced night vision that could see people moving miles away and remote monitors that could detect people walking in the open desert. One can only imagine the technology at their disposal NOW. I find it hard to believe a wall could efficiently secure the border anywhere except high-traffic areas near major border crossings (e.g., San Diego has one).
GIVEN THAT WE CANNOT STOP ALL ILLEGAL IMMIGRATION, I definitely want the border as secure as is REASONABLE. In other words, I want cost-effective security and a reasonable trade-off between reducing illegal flow of people and impact on natural resources.
If building some segments of wall in high traffic areas is a cost effective solution, that seems reasonable to me; but that sure doesn't seem to match the ridiculous nonsense Trump and his supporters keep spouting.