The club do make gestures re: pricing/tickets regularly. Remember offering ST holders free tickets for the Hull City replay last year ?
http://www.seagulls.co.uk/news/article/albion-fans-treated-to-replay-tickets-1367144.aspx
And they're always flexible re: pricing for cup games depending on the opposition. Cheltenham was 12 quid earlier in the season.
But like it or not Arsenal are in that top 6 of clubs in the entire country in terms of opposition. (Man U, Man City, Spurs, Liverpool).
The game will certainly sell out.
And of course who could forget the free pint at the Middlesbrough (was it?) game.