Once you get there you could find even outrageous the price you need to pay to move around London by public transport. A single tube ticket is in fact worth a staggering £4, but you can (or rather, are suggested to) consider a couple of other available options.
If you are just going to London for a one-off trip, then you are strongly suggested to buy a Travel card, you can check in the Transport for London website for details about the different fares www.tfl.gov.uk.
Nonetheless, take note that if you should buy an Oyster card you’ll never spend, on any given day, considering the different zones within you are moving, more than you will be spending if buying a travel card. In other words, Oyster card daily expense is capped to a daily maximum, which is fixed to the same amount you’d have spent to buy a daily travel card. So that, in case you are using the Oyster card on a pay as you go single fares mode and you should use it for a large number of times, the maximum amount which will be detracted to your total credit won’t be higher than the cost of a daily travel card, considering, of course, the zones you are travelling to and from.