From Hong Kong Free Press:

MTR Airport Express to increase fares for the first time in its history in June

Hong Kong’s Airport Express MTR line is set to increase fares by 10.3 percent in June – the first fee hike since it came into operation in 1998.

According to a Tuesday Legislative Council document, fares for journeys between the airport and the city proper, paid using an Octopus card, are set to rise by HK$5-HK$10. [..]

The MTR has never adjusted fees for the Airport Express, except in 2000 and 2001 when it cancelled discounts introduced at the time of the line’s opening.

Well, that’s not quite true.  Last year they increased the prices for the group tickets by around 12%, as noted here (Hong Kong not so good–Airport Express).