Yes, the grocer's apostrophe (do a google) is very irritating.
The curious thing which I always ask myself is how one can write that way, having read any English books
Back to the topic, I can't see a rational basis for saying £20 is taking the micky. Sure, it could be lower, but you probably paid £100-200 to get there, and the difference to the airfield is huge (a factor of two actually
).
Experience around the UK tells us that £20 delivers a much better maintained place than £10. Obviously much of it is down to management etc but £10 is evidently hard to make things work at.
Private strips can be free, etc, and there is a vast pilot community which will absolutely not pay more than £5 (they must fly mainly between strips) but then somebody is subsidising it by cutting the grass etc in their own time, which is lovely but not substainable.