The Taxpayers' Alliance, which is nothing to do with the government despite sounding boring enough to be so, has run the numbers on legalising cannabis. They say it'd work out very well.
They say, in fact, that the country would save a very precise £891.72 million each year were drug laws to be relaxed and marijuana to be racked alongside the plain-wrapped Marlboros, with the savings coming from many unusual sources, such as lower prescription costs because people might self-medicate the pain away, and less spending on policing for keeping tabs on low-level drug crime.
The full report [PDF] explains that one million police hours are spent on manning cannabis enforcement alone, with thousands of users held in custody each year and costing forces millions in putting people up in cells for the night. Courts would save millions, the CPS would save millions, prisons would save millions, and there's even a knock-on effect for people who end up with criminal records because of some misguided dope hijinks -- take these away and they may be able to find better jobs in the future.
Twitter might even be nicer if everyone's stoned, too. [Taxpayers' Alliance]