The prices for 2015 have been announced by TfL etc.
Some significant changes that affect the way many people here plan their trips, and advice given previously.
For example, the cheapest paper one day travelcard covering the central Zone 1 area will be £12. However, the 7-day Zone 1-2 travelcard has 'just' an inflation increase to £32.10. Therefore you may only need to be in London for 3 days for this to be cheaper than one day cards, if you are buying these at a railways station for the 2-for-1 offers.
On the other hand, daily caps on Oyster PAYG use have been reduced, for example in Zones 1-2 to £6.40 per day.
Since 25% of Oyster cards are refunded within a month and this churn is considered a waste of money, there will now be a forfeit of £3 of the deposit fee if an Oyster card is returned within 1 year of purchase.
There will be some kind of short term discounted version of the Oyster for the under 16s, recognising that the current application for a Zip card is onerous for a one off child visit.
There are many other large swings and roundabouts for people particularly in outer zones but these by and large are less used by visitors.
If the above all reads like gobbledegook don't worry. People are paid to come up with this stuff ... and the help here is free.
Full info here: https://www.london.gov.uk/sites/default/files/MD1418%202015%20Fares%20for%20submission%20%28signed%29%20PDF.pdf