The price of Gold OA today is absurdly, arbitrarily high.
Most journals (and almost all the top journals) today are subscription journals. That means that whether you pay for hybrid Gold to a subscription journal or for "pure Gold" to a pure-Gold journal, double-payment is going on: subscriptions plus Gold. Institutions have to keep subscribing to the subscription journals their users need over and above whatever is spent for Gold.
In contrast, Green OA self-archiving costs nothing. The publication is already paid for by subscriptions.
So it is foolish and counterproductive to pay for Gold pre-emptively, without first having (effectively) mandated and provided Green.
(That done, people are free to spend their spare cash as they see fit!)
So what RCUK should have done (and I hope still will) is to require that all articles, wherever published, be immediately deposited in their authors' institutional repository -- no exceptions. (If it were up to me, I'd allow no OA embargo; but I can live with embargoes for now -- as long as deposit itself is immediate and the email-eprint-request Button is there, working, during any embargo: Universal immediate-deposit mandates will soon usher in the natural and well-deserved demise of OA embargoes.)
(That done, whether or not authors choose to publish or pay for Gold is left entirely to their free choice.)
Paying instead for Gold, pre-emptively, for the sake of CC-BY re-use rights , today, is worth neither the product paid for (Gold CC-BY) nor, far more importantly, all the Green OA thereby foregone (for the UK as well as for the rest of the world) whilst the UK's ill-fated Gold preference policy marches through the next few years to its inevitable failure.
So it's not about the price of the Gold. It's about the price of failing to grasp the Green that's within immediate reach today -- the Green that will not only pave the way to Gold (and as much CC-BY as users need and authors want to provide), but the same Green whose competitive pressure will -- (here comes my unheeded mantra again) -- drive the price of Gold down to a fair, affordable, sustainable one, by making subscriptions unsustainable, forcing publishers to cut costs by downsizing, jettisoning the print and online editions, offloading all access-provision and archiving onto the Green OA institutional repositories, and converting to Fair-Gold in exchange for the peer review service alone, paid for out of a fraction of the institutional subscription cancelation savings windfall.
The difference between paying for Gold then, post-Green OA -- and hence post-subscriptions and double-payment -- and double-paying for it now, pre-emptively, is the difference between Fair Gold and Fool's-Gold.