Of course, inflation changes the figures but I would roughly say there are three price points.
£25 - £30 = Drayton Manor, Flamingo Land, Lightwater Valley, Oakwood, Parc Asterix
£35 - £40 = Alton Towers, Thorpe Park, PortAventura, (most) Six Flags parks, Dollywood, Knott's, Busch Gardens...