What is the cheapest way for me to travel? (Trains-related)?

You cannot use an Oyster card on the routes from Sunningdale - see :- a href="http://www.tfl.gov.uk/ets/downloads/tickets/Get_most_out_of_Oyster_Card.pdf" rel="nofollow"http://www.tfl.gov.uk/ets/downloads/t…/a and scroll down until you reach the part about National Rail lines that accept them. You will need to check the prices of a 3 month/sixmonth and annual season from Sunningdale to Waterloo. It might be cheaper to buy the annual and then surrender it when your travel time is up, loosing any admin fee. You can get an Oyster card for the journey Waterloo/Chancery Lane linked to a long period travel card.