Sid, the drop in migrants almost certainly has nothing to do with a boneheaded plan to waste 15B on a wall that is easily scaled with a $100 ladder and everything to do with obnoxious rhetoric.

Studies purporting to show a cost-benefit analysis for migrants are all over the map.  Some right wing think tanks say costs are as high as 100B per year while others dispute that there is any net cost at all or point to the economic benefits created by an underclass that pays taxes but cannot avail themselves of such things as Medicaid, Social Security, or unemployment insurance.

I don't think anyone has a reliable answer -- but that 15B would go an awfully long way in improving health care, education, or usable infrastructure.  Surely of more benefit than an ineffective symbol that really only buys Trump an opportunity to avoid eating crow.