The Keep Britain Tidy campaign have estimated that the cost of graffiti removal to local authorities at £27 million a year.  They also point out that over half of councils don't have a dedicated team to deal with graffiti.

I know that there's much more we could be doing on graffiti here in Lewisham, but the vast majority of the stuff I see around and about is on private property - businesses in particular seem to be very hard hit. 

If you add the tagging and scratching that goes on to public transport the cost must be at least double the amount that local councils are faced with.