Historic Farnborough public-day prices per the good old Flight archives:
1950 3s weekend ( no public on Friday )
1960 £1 Friday, 5s weekend
1970 20s Friday, 10s weekend
1980 £5 Friday, £4 weekend
1990 £11
2000 £15 in advance or £20 at the gate.
Additional per-vehicle parking charges applied although in those days I reckon most people arrived by other means.
It would be interesting to align those with contemporaneous average daily pay.
Nowadays a single-day ticket for RIAT is £55, so Farnborough sadly seems to be in line with the cost of 'large UK airshow' these days.