It really depends on the tour, artist, what size venues their playing and production costs. I'd say for standing tickets anything form £15-£60 is fair.
I'm not big on concerts, but because I do not attend them very frequently (and when I do attend a concert it's usually because I absolutely adore the performer), I usually don't mind paying an absurd price for tickets.
Anything more than about £30 and you'd have to be Katy Perry for me to consider buying tickets, and that's only because there's a chance I could meet her again.
Anything more than that and your show is clearly too big and extravagant for me. Simple shows >>>