You're so right that in the UK we now have officials which measure and estimate how much hookers and drug dealers add to the economy - and its a LOT.
There is a fascinating BBC article on the data:
Extrapolating from research for 2004, the ONS estimates that there were 60,879 prostitutes working in the UK in 2009. Based on Dutch research they assumed that each one serviced 25 clients a week, with an average price per visit of £67.16.
What's important for the measurement of the national accounts is the margin taken by dealers, except in the case of half of cannabis consumed in the UK, which is assumed to have been grown here.
The ONS took figures for drug sales from a one-off Home Office survey of drug use in 2003, which gave them an average amount of drugs consumed per person. It took retail prices from a UN report and adjusted for purity using evidence from seizures by police and border agencies. Comparing this value with the UN's wholesale drugs prices gives the margin that the ONS is interested in.
Each year's figures for demand are derived from the number of drug users shown in the Crime Survey for England and Wales. That gives a figure for 2009 of £3.6bn for drugs other than cannabis and £830m for cannabis.