Spa tickets have consistently dropped in price for about the last 15 years. First time I went in 2008 I paid £150 for GA, its now considerably less. Last year we for chatting to a guy from Germany who has been to Spa every year since he can remember and he's confirmed its always getting cheaper.
Silverstone has always been about £150 for GA (since I started going in about 2003), but the developments mean than GA areas are only a fraction of what they used to be, and grandstand prices have shot up. So overall that has got more expensive. But its because they are having to recoup the money for the investors that funded the development.
Winner is Malaysia. General admission is like £7 or something stupid!