How much does it cost to spend a week in Thailand?

I'm going to visit Thailand with my friend at the beginning of June. We are going to spend a couple of days in Bangkok and see as many as we can "must see" things there. Then we are planning to spent 4-5 days in Koh Samui. Could anybody, please, tell me how much money we need if we are oriented to cheap 'backpacking' travel. No luxury hotels, no expensive restaurants.