There are several different ways to buy an Oyster card at different prices. You can get a pay as you go card, a season card etc and there are discounts and special offers available. The price also varies according to whether you stay in the Zone 1 areas near central London, for example, or want unlimited travel all over the city; and whether you travel at peak times.

For example, if you want to travel around central London for 1 day, a Zone 1 off-peak card will cost a maximum of £7, or peak travel costs &8.40. Prices are similar for the different zones, but if you want to travel in all 6 zones it is &8.50 for off-peak travel.

If you are staying for a week, you could get a 7 day card and that would cost between about £30 and £55 depending on whether you want unrestricted travel in all zones.
You can get more detailed price information here.