With regard to City fans, they've been there before, on numerous occasions, including this year and it is a semi final! I would compare it to the fact that the SF is our big fish and we are struggling to sell out a London game against a top 6 team!
Apologies if it has already been asked but is there a reason why you won't purchase a membership if it enabled you to get a ticket for the semi? It's £25 and therefore not a great amount in the grand scheme of things, ie costs for going to the semi (and potential final).