The quantity of such under the table business is difficult by definition to ascertain. If the IRS claims that one out of six people has at some time in their life gotten income dealt under the table, that might be believable. But that many people doing it all the time? Pulling my leg.
According to the IRS, most tax evasion occurs where there is no third party reporting and can be as high as 50% among that group.
While some see the national retail sales tax as the answer, estimates of tax evaision for that scheme are as high as 1 in 5.