Many of these workers are being paid 'under the table' and pay no income tax. Still it doesn't matter. Illegal immigrants are just that-"illegal". The wall will go along way to protect our southern border and help makes us a nation of laws again. And before you ask I do feel for those outside the US that are downtrodden, but that's beside the point. BTW, over 5 billion was appropriated for the wall construction during the Bush administration but it was never spent; so we already have a downpayment in the bank.
Under the table payment doesn't explain the billions in taxes coming through fake socials even immigration says is from illegal migrants. It's a lot easier for both undocumented migrants and employers when a fake social is involved.
As for the border wall being in any way effective, there's simply no evidence of this. Once again, the vast majority enter legally on temporary visas and overstay, and don't enter on foot. This does nothing about planes, ships, tunnels, or cars where the occupants already have visas.
Incidentally there's 5 billion because Bush's Secure Fence Act was a colossal failure. It got mired down in very real considerations for habitat protection, Native American site protection, property ownership laws, etc etc. Ended up being so expensive once the research was done that it was abandoned. Which is funny, since Trump just announced the wall is going to be more expensive than they thought. Big shock.