For the UK proposal, the Guardian article from July 15 [1] mentions article processing charges (APC) around GBP 2,000 per article, presumably paid by the institution to some private institution to manage the peer-review process. Of course, the peer-reviewing itself is done free of charge by fellow academics. The Nature post doesn't seem to address this point (unless this is what they refer to as "up-front publication costs" in the second paragraph).
Either way, charges around GBP 2,000 per article for processing are hefty (especially given that the bulk of the work is done by the academics for free). Maybe one should ask the question how this figure could be driven down?
[1] http://www.guardian.co.uk/science/2012/jul/15/free-access-br...