Most of the tickets will sell once the prices drop.
Except they're not going to drop in places like Pittsburgh or Louisville where a solid number of GA tickets haven't sold. I don't see U2 dropping the price on those--scalped tickets, sure, but not unsold ones.
However, maybe a Groupon sale might work?
U2 doesn't have any control over the prices on secondhand websites like Stubhub; that's dictated both by the market, and the individual seller. On the 360° tour, I remember some US cities being slower to sell (though they did ultimately sellout), and I was able to buy tickets at dirt cheap prices, gifting them to friends as Christmas gifts. Seriously, I remember buying a batch for the Georgia Dome in Atlanta for something like $10 a piece.
So long as a show isn't soldout, you can many times find tickets for UNDER face value on websites like Stubhub. Take the current Radiohead tour for example. Seriously, go and look at prices on Stubhub for their show tonight and their next one. They're down to $20.