Apologies if this has already been answered elsewhere.... but if I
were to use a PAYG Oyster card between Balham and Victoria, or Balham
and Shepherds Bush, how much would I be charged? And would it be
cheaper for me to buy an ordinary National Rail return fare if I was
doing a return? (seeing NR turn-up-and-go returns are approx the same
as a single).

Thx
Phil