With Sri Lankan foods and local transport, and Budget range hotel around 800 to 1200 a night, I would say 400 to 500 US$ will do. But be mindful that tourist attractive place entrance tickets are expensive, and there are many tourist traps too.
A days food cost can be managed around 500 to 600 LKR, if you can manage with local foods and walk in to local food shops.
If you like to spend long time then avoid beach sides and touristic places then you be saving a lot. May be like Badulla, Ella, Passara. But then its hard to find hotels here but some modest guest houses or may be rent a house or room can be found. about 6 years ago
It depend about your budget and the requirement.If you are comfortable with basic facility accommodation and public transport.You can save lot.but its bit hard and difficult.because as an each and every country there are lot of local people try to cheat you and tell double price to you.we had same experience when we at Thailand.I think you better contact a travel agent in sri lanka.they will arrange accommodation,transport and other facilities as per to your requirement.if not you can contact a friend who is living in Sri Lanka.If you want more details about sri Lanka.feel free to contact me anytime. about 6 years ago
This site has some good information on both costs and access to money http://www.tourslanka.com/sri_lanka_travel_faq.htm almost 6 years ago