I'm gonna sound old but running shoes are way more expensive than they used to be! I'm sure there's already been a knock-on effect of Nike realising that people are willing to pay £200+ for a snazzy pair of shoes with springs in them, so all of a sudden £100+ doesn't seem quite as much for the normal ones. I've still never paid more than £100 for a pair but the costs are edging up even for online bargains, plus it's probably worth paying a small premium to be able to try on a few pairs in an actual shop and get advice. Once you've found a pair you like you can usually get them (or last season's version) cheaper online... till the manufacturers inevitably discontinue them. Bah!
Re: mileage, the 500 mile rule is one of those that has an element of truth, but is partly sales guff to make you buy a new pair when you probably don't need to, and also varies a lot between different people, according to how you run, how you land, what bits of the shoe wear quicker than other bits, etc. I'm not particularly light on my feet but can usually get closer to 1,000 miles from a pair, and often more like 1,500. The limiting factor is usually the outsole wearing away to the foam (usually outside of the heel cos I'm a heel-striker.) But you might find the spring in the midsole going before then. I think it's very much trial and error, but don't bin them just because they've reached a certain mileage.