If one catches a train to or from either of the two airport stations, there is a station access fee of $14.30, capped at $29 per week, which is in addition to the normal ticket price.
There is some history behind this. The airport stations were privately funded but patronage was lower than expected and the company, Airport Link Company, defaulted on debt repayments. The contractual arrangements were then reworked. The contract between the company, Airport Link Company, and the NSW Government is commercial-in-confidence, in other words not available for the public to look at. However, some information is available:
The above information, apart from current prices, is summarised from the 2014 NSW Parliamentary committe report titled: "Removing or reducing station access fees at Sydney Airport".