Tim Berner-Lee's free lunch
It may be "more expensive to store [data] somewhere private" if all you're doing is sticking say an Oracle dump file somewhere on disc.
However to actually publish data in usable form one has to re-format it. It will also need to be sliced and diced in ways one would not if it were just sitting on your local database. It also needs version control and/or updates too. Above all it must be documented.
Then you need to set up a library and indexing system. (Google isn't going to help much with a few thousand gigabytes of delimited text files sitting on your Open.Gov FTP site).
Not looking nearly so simple or cheap now is it?
TBL is probably used to hordes of eager undergrads and PhD wallahs doing this for free in his World. Well the Civil Service isn't cheap labour and probably can't, aren't or won't be doing it anyway.
So here's another rich vein to be tapped by the usual suspect management consultancy vampires. I'd like to be a bit more certain what benefit will accrue beyond another Government PR stunt.