No idea what it costs, but GDS[1] is doing a pretty good job on http://gov.uk. My guess is that only the savings on closing hundreds of sites [2] justifies it. Maybe it also prevents some of the uk citizens from not going nuts while trying to do gov business online ;)

[1] http://digital.cabinetoffice.gov.uk/ [2] https://www.gov.uk/government/news/hundreds-of-government-we...