Seems to be the higher the entry cost, the larger the entry field you get. If a particular race with a £30 plus entry fee reaches entry limit then it does send out the message that that is what people are prepared to pay.
What works for me is to mix and match the races -parkrun or Halewood 5k which are free Saturday races, mid week 5k, 5 milers and 10k club organised races in the summer which are less than £10 or even less than £5. Then throw in a dozen or so of the longer Sunday races steering away from the commercial ones unless one is particularly appealing.
You do get to race regularly though not all at your preferred distance. This year I reckon 65 races will have cost me £200.