The “tax gap” is the spread between what the government thinks it should collect in taxes and what it actually collects. In 2001, the tax gap, after enforcement activities, was $290 billion. In 2006, the most recent year for statistics, the tax gap grew to $385 billion.
Compliance with tax laws has declined. In 2001, compliance was at 86.3%. In 2006, compliance declined to 85.5%. Compliance is highest when there are third-party information returns. For example, compliance with wage reporting is at 99% because employers issue Form W-2 to employees.
Source: IR-2012-4
What a willing buyer would pay to a willing seller when neither is under any compulsion to buy or sell.