My eldest and his girlfriend have weekend tickets for Silverstone. It's very expensive.
Last year they were able to go Thursday free of charge and we saw them on the F1 Show.
This year they are charging I believe, but they entered a draw for free tickets and managed to get them.
A friend was trying to book tickets and they could see the prices rising as they progressed in the online queue. By the time he got to buy the tickets they had risen so much he didn't bother. Its called dynamic pricing I believe, price increases with demand.