Cash fares are £2 for each bus journey for an adult.
If you use an Oyster card, only 90p is deducted for each bus trip up to
a maximum of £3 per day.

You will almost certainly save money if you buy an Oyster Card.

London's fares are complicated; you can read about them on www.tfl.gov.uk
Single cash fares are usually much more costly than Oyster/One Day
Travelcard or a One Day Bus Pass.