You can't buy a ticket without being at least red membership. Tickets are sold out fast. If you have a membership, you can scout Arsenal Ticket Exchange and look for tickets released by someone who is not able to attend the match.

Or head to @arsenal_tickets on Twitter.

It depends to match category. They categorized their match with A,B and C. High profile match like vs Man. United or NLD is categorized as A match, hence the ticket is expensive even for low tier seatings.

C grade match like vs low tier club eg: Bournemouth is the cheapest. Usually someone will sell the ticket at face value price. If I'm not mistaken it was around £38 - £40. I tried to purchase a ticket vs Stoke last year and found someone is selling a seat behind the corner flag at £38.50 in TicketExchange. Unfortunately my credit card TAC was sent to MY number

You can refer here: http://www.arsenal.com/tickets/matchcategories