When I was a serious runner, I bought new running shoes every 500 miles. The newly retired pair would be sent to the casual use pile, and I'd throw away the oldest pair. The retired shoes were fine to walk around in and do regular stuff, but no longer up to running.
I don't run anymore, so I bascially wear sneakers until the wear out, but I can tell the difference when I do get a new pair. I don't buy expensive runners anymore, though.