Sri Lanka is expensive in some ways you might not expect. It appears to be inexpensive in some realms but it's the unexpected things that will get you like the extra charges incurred. You might rent a room really cheap but if you want access to the facilities, you'll have to pay more for them.
The same is true when renting. You might rent a room or a whole house cheap but then incur unexpected charges for using things you'd think would be included. Taxis may be cheap but tips are expected to be high enough to compensate the drivers for the cheap rates. It's the unexpected things in the country that will surprise your pocketbook and not in a good way. Expect the unexpected and you'll be fine.