Whatever the government may say, there is no way the cost will be contained to the current official budget of £9.3bn - let alone the preposterous £2.1bn they originally plucked out of the air (or somewhere) in 2004. Our estimate of the final all-in costs has always been £20bn (eg see here). We see no reason to depart from that.
2. Prison - it's still cheaper to bang 'em up
The Sunday Times has the useful graphic above which summarises the relative costs of leaving criminals free to roam our streets:
"The average number of offences committed by a single criminal is 140 per year, according to a Home Office survey.
Civitas, the think tank, estimates the cost to society of one active criminal committing 140 crimes is £280,000 a year, while the cost of reprocessing him or her through the criminal justice system and back into prison is put at £65,000."
And once they're inside, the average cost of a prison place is put at £26,000 pa.
Now you don't need to be a calculating machine to work out that it's a lot cheaper to lock up bad guys than to leave them out here.
So WTF don't our useless politicos JUST DO IT?
3. Buying votes with welfare