The 16-21 year olds price works out dearer than the same age band season ticket price if divided by 19 league games. So you pay more to see two reserve sides in a cup that no one is that arsed about being knocked out of than you would a title deciding match in a Manchester derby :-/
£22.50 is a good price if all matches were at that price, but with Arsenal two days before, Fulham few days after and many doing Madrid, Amsterdam and Dortmund it's a lot when it's all added up.
Villa will bring 3-400, we will have 18,000 or so, it will be less than 20,000.