It really depends on each seller but most of the time the going rate is £1 across the board, which obviously not always is a good deal per se so if the release is really good i'm happy to pay the asking but most of the times i try to haggle.
You do see some people letting them go for 50p. The trick with carboot's probably is to buy in the last hour or so as at that point many people are wanting to clear as much as possible from what's left and you start seeing deals like 4 for £1 and even lower.
I managed to get 66 DVDs from a few sellers for 10p each and then i had several deals based around 4 for £1 price.
Basically got a lot of films for fairly little money.
Have to say never bought any films from CEX so will have to have a good rake around the place next time i'm in there, which is rather often.
Yes, i'm still in Inverness so let me know when you are coming this way next time, maybe we could catch up for a pint.