Coming from Gatwick Airport, an off-peak day travelcard route Thameslink Only is
£18.30 (and a super off-peak for Weekends only is £15.20), while the off-peak daily cap on Oyster/Contactless is
£23.70. It's been the case as long as I can remember that it's a fair bit cheaper to get a paper Travelcard if you're going to use Thameslink into London, and even if you want to go to Victoria, given the saving it is often worth changing at East Croydon for a fast Victoria service.
For a single journey, PAYG does seem to be cheaper than a paper ticket though.