Well, let's see here. We pay ~1 E+4 USD per year in cash to various people for cleaning, home repair, lawn care and so on. I strongly suspect that they don't declare that income. Multiply that by ~114 E+6 US households, and you get ~1 E+12 USD per year. That's about 10% of estimated 2010 US personal consumption expenditures -- 10.4 E+12 USD per year, according to the US Bureau of Economic Analysis
. Even discounting for honesty, that's a large share. And we in the US are probably more honest about such matters than most, yes? China? India?