By which I mean "getting people who don't normally go to the theatre* to go to the theatre", and not "compliance with the W3C's AA standard". Audience Development, in other words.
* Please replace "Theatre" with your art form of choice: Gig, concert, ballet, mime, opera, site specific devised physical community installation, etc.
Also, this is more opinion-y than analytical. And they aren't very rigorous opinions, and I'll make some sweeping generalisations which will certainly be wrong in some respect.
My starting point is a set of feelings and opinions that I believe many of my friends and people like them share:
* That theatre (slash "your art form of choice", I'll stop doing that now) is important and that if more people went to the theatre the world would be a better place.
* That not enough people go to the theatre
* That theatre is not and should not be for the posh people only
* That £10 tickets to the national theatre will somehow help this
* That they, as impoverished theatre makers, who need no encouragement to go to the theatre but aren't rolling in cash, are particularly keen on the £10 tickets and take advantage of them as much as possible, is pure coincidence
In short, that discount or subsidised cheap theatre tickets are important because they broaden the reach of the audience, conveniently ignoring the fact that discount or subsidised theatre tickets are mostly bought by people like them, who are very much in the choir (which is, in some sense, and this metaphor is horribly mangled, being preached to).
I think that access to the theatre is almost nothing to do with ticket prices, and therefore that allowing ticket prices for popular or sold out shows to float to their real market value, allowing promoters and venues and performers to make as much money out of them as they can, would not have undesirable side effects of making the art form intrinsically elitist. It's quite easy to find indisputably non-elitist art forms (let's pick on the X-Factor live, for example. Or football in general) that have ticket prices that seem to me to be massively out of proportion to the value of the performance, that sell very well indeed.
All of the people that I know who love theatre do so because they've been involved in making it. Whether they are now still working as actors or directors or lighting designers or critics, or have got other jobs and spend money on tickets, or have even gone to the trouble of sinking a couple of years of their lives into writing a new box office ticketing system from scratch (Monad Ticketing, thanks for asking), it starts with getting involved, not with cheap tickets.
So the subsidy - and selling a ticket you know you could get £50 for for a tenner is a subsidy, no matter where the money comes from, is in the wrong place. Instead, it should be going into making it easier to put on a show, less of a risk to put on your own little fringe thing whether or not you've been to drama school, cheaper to rent a space above a pub and try it out.
* Audience development is best served by subsiding the costs of putting on a show, not by subsidising ticket prices.
* Popular shows should implement dynamic pricing in order to make as much money as possible, and if this means that no tickets for popular shows are available in the lower price bands at all, that's not a problem.
* Unpopular shows should implement dynamic pricing in order to make as much money as possible, even if the last few seats are filled for next to nothing, because there is a certain price point at which people will go "well, we might as well see what it's like"