Do not pay over face value for a ticket at any stadium sized show. Theres no need to. The internet is full of wannabee scalpers who buy tickets for a show they think they can make money on and who end up getting their fingers burned.  Its simple supply and demand for a show of this scale where tickets are already (for the most part) overpriced.

Viagogo is sound enough I think, but only use sites like that if youre really desperate. And do so at the last minute when they run the risk of losing their money completely.  There were gold circle tickets for Dublin on viagogo on Wednesday night for half price.

If you just turn up on the day and be patient, I think you'll get what you need for pretty much any show.