I stumbled onto this video, of all places, on the yahoo finance page Should You Save or Splurge on Running Shoes?
Bottom line is they recommend that you splurge, and I could not disagree more. For the most part, I've been able to stick to the $50 or less for running shoes rule for as long as I can remember, with few exceptions. I routinely look for shoes marked clearance, discount, discountinued, 20% or more off, sale, you name it.
My guess is it's more important for you to run more off-road if you want to reduce injury.
Check out one of my posts from 2009 Why Expensive Running Shoes May Be a Waste of Money.