This is fairly straightforward – unless you work at the airport or for a train company you can’t get discounts on the Gatwick Express.
It is the quickest and comfiest way to reach Gatwick from London Victoria, but remember that Southern Railways also goes from London Victoria, and Thameslink from many stations in the City – both for at least a 1/3 saving if you book in advance.
However, if you do want to book Gatwick Express Tickets, the cheapest way is via their own website as they offer online discounts, and occasional special offers.