Hi exiled claret, I agree totally with you on GNR, I so want to do that race but will not spend £50 on a 1/2 mara full stop. VLM at £28 entry is extremely good value,only shame is the ballot system but that's another thread.
Yes there is a rise in 'big' races across the country, but in my opinion this just adds to the choice and for some will reduce travel costs, i've done about 10 events this year and I make my choice based on cost as much as reputation, location numbers etc.
I do feel there are a wide range of prices avaliable within each race type for all pockets as has been mentioned, they may not have closed roads, heavy advertising/sponsorship, big crowds, gels/refreshment along the way etc but that comes at a price.
Sprint for the line, I think the market is becoming over saturated with the 'big' race with all the bells and whistle and a nice steep entry fee, but these are heavily promoted and seem to maintain their numbers. The Bupa series events is a great example the London 10k is around £30, GNR (1/2 mara) around £50 both these races continually sell out which I find amazing but each to their own.