This is by design; for quite a few years there was a notion in sentencing policy (and similar areas like this) that the best deterrent effect was obtained my multiplying the social cost of the offense by inverse probability of detection; that is to say, there is a fairly low chance of being caught but the people who do get caught are punished for both their infraction and that of all the people other people who weren't caught.
Essentially criminal justice today uses the language of morality (very popular with conservatives (qua Jefferson Sessions, who as an elf^H^H^H attorney-general is apparently magically immune from punishment despite his perjurious habits), but uses the logic of economics and incentives. The administration of criminal justice is for all practical purposes a commercial enterprise built around slave labor which depends on a steady supply of incarcerable people to function profitably.
A full discussion of this is beyond the scope of a HN comment or even a single article or book; entire careers have been devoted to the study and analysis of this issue. This law review article offers a somewhat accessible introduction.
http://www.masonlec.org/site/rte_uploads/files/Hylton_crimec...
EX: Bike couriers are a good example of relatively high pay and often minimal documentation.
Again, I don't think you could knock the ball out of the park or allude a motivated state agency if you were a terrorist or something; but if you were in a similar position to the subject of the article you could probably avoid capture and maintain a semblance of normalcy.
You could also become a loan shark, which can be lucrative, but your illegal status would give your customers leverage against you, which is something you really don't want if this is your retirement savings...
Becoming a loan shark is a possibility, I suppose, but puts someone in direct competition with professional criminals and creates a whole new set of existential and legal problems.
One low-risk strategy I've heard of being used by cash-savvy people is purchasing food stamps at a discount to their face value. Again, pretty much any kind of black market activity comes with legal risks.
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.