Yes, Audix tends to sell for a bit cheaper, but it's only about $10 less on average. Not enough to really make a difference for the bigger sound companies (ie, the guys who have enough gear to do festivals).
No, the trend is changing because Audix tends to have a flatter response than the corresponding Shure model. The SM57 and 58 have a low-mid bump that has to be EQ'd around, whereas the Audix doesn't.
All across the price levels, Audix tends to have microphones that sound just a little better than the Shures, typically with a little less handling noise and better feedback rejection. And, because they don't have the SHURE name plastered on them, they tend to still sell for cheaper. THAT'S why the big guys buy them.
The club guys buy Shures because they don't know any better. It's pretty rare to have a knowledgable dedicated soundman in a club, so they buy what's been working for the better part of the century. OR they're like me and had already bought their Shure gear... As soon as my Shures start to die, they'll be replaced with Audix.