"Within this framework, the EMCDDA has calculated a range of estimates of public expenditures on drug-law offenders in prison. The low estimate considers only those prisoners who have been sentenced for a drug-law offence. The high estimate also includes pre-trial prisoners who may be sentenced for a drug-law offence (assuming that the proportion of drug-law offenders among pre-trial prisoners is identical to that of drug-law offenders among sentenced prisoners). Applying these low and high estimates, between 2000 and 2010, public expenditure on drug-law offenders in 22 European countries is estimated to have been within the range of 0.03 %–0.05 % of GDP. With the exception of the first two years, when the number of countries with available information was limited, these proportions of GDP remained stable. When applying these percentages to the whole EU for the year 2010, public expenditure on drug-law offenders in prison is estimated to have been within the range of EUR 3.7 billion to EUR 5.9 billion.
"Over the period 2000–10, it is estimated that 12 out of the 22 countries spent on average between 0.01 % and 0.03 % of GDP on drug offenders in prisons, if we account only for expenditure on sentenced prisoners. If public spending on pre-trial prisoners is included, then the estimates exceeded 0.03 % of GDP in 10 countries, reaching a maximum of approximately 0.08 % of GDP in 2 countries."

Source

European Monitoring Centre for Drugs and Drug Addiction, "Estimating public expenditure on drug-law offenders in prison in Europe," EMCDDA Papers, Publications Office of the European Union, Luxembourg, February 2014, p. 14.
http://www.emcdda.europa.eu/p…