Considering taxes for more conventional jobs, $40k under-the-table cash would be the equivalent of $60-70k of conventional wage income. Depending on city and state, that would be either solidly middle income or lower middle income.

First, many undocumented immigrants pay state and federal taxes; it's difficult to obtain a bank account otherwise. Second, their exclusion from the regular system of credit scoring and numerous other aspects of the economy that most people take for granted impose a significant hidden tax, albeit one that mostly flows to private actors such as landlords etc..

I'm not sure how that relates to my comment.

I was replying to the idea of $40k (cash, untaxed) as not much money. It corresponds to the take home of a middle class income in much of the US.

My point is that it's not really 'untaxed' in practice.

Even just 10$ an hour is good pay for under the table work. 40k is well past that.