I'm certainly only a very infrequent visitor to the AVP. However without doubt its the best such facility in the UK and as such from a purely personal point of view I would not mind paying that sort of money. But I agree for those with a casual interest or those who want to visit regularly it is expensive.
With respect they may be dreary modern airliners to you, but others may think differently and as both a spotter and photographer there is still plenty there to keep me interested.. Sadly however I live too far away to warant visiting, other than when I happen to be in the area.
Having said that in days long gone (late 70's-early 80's) I used to pay around £10 to park my car at Heathrow for a days spotting. The £12 at MAN suddenly doesn't seem too bad!