You might be thinking something like Amersham to Shenfield which is £11.00 in the peak time, or perhaps you’ve remembered that Gatwick Express now uses Oyster and charges £19.80 single. They’re both good guesses and neither can be beaten with a single paper ticket, but the Gatwick Express one is more expensive than paper if you make a return.
However, say you’re in Stratford and want to see a friend off from the airport. You go to Westfield for some last minute supplies and then want to travel. One of the ways you can do this is to take Southeastern High Speed to St Pancras then Victoria line to Victoria and finally Gatwick Express. Journey time is about 1 hour and avoids the slow Thameslink crawl through south London thanks to the London Bridge rebuilding. Once you’ve seen your friend off you want to come back to Stratford. How much will this cost?
Stratford Int’l to St Pancras Int’l £3.80 (off-peak)
Kings Cross St Pancras to Victoria £2.40
Victoria to Gatwick using the Express £19.80
Total one way: £26.00, both ways £52.00 assuming off-peak on HS1
or Stratford Int’l to Gatwick Airport off-peak day return £23.30
So that makes Oyster 123% more expensive.