Hickstead on Derby day is £28 this year plus £8 for parking (fairly sure it was last year). Badminton on xc day at the gate is £27 plus £12 for parking this year.
Windsor really isn't 'so expensive' compared to most other big events.
See I don't as equestrian sport is relatively cheap compared to other international level sports. You pay £90 to watch England play rugby at Twickenham, slightly less for Wimbledon Centre Court tickets and don't get the shopping and exercise from walking around either!
You get the evenng performance included in the price, so I think that's very good value for money really!
Wednesday is free and Thurs and Fri tickets are less than £20