I take it the locals can park for free, which is obviously how it should be.
So for your fiver you get to drive into a world class monument, park your car and are then free to walk around the monument unhindered. That doesn't seem too outrageous to me. Whats a fiver? a packet of fags, a couple of drinks in the Red Lion. If there's four of you in the car then that's only one pound and twenty five new pence each.
I would hope that there's a decent bus service to the village and that this charge may persuade more folk to use it. I drive a car meself so I'm not gonna start preaching but there are really are too many cars on the road and Avebury is only a small village.

I guess I'd better put me tin hat on.....