I am from Sri Lanka. Except for the rent, all other expenses are pretty much the same in every city.
The two major cities are Colombo, the capital, and Kandy. In Colombo the rent would be around 20,000 Rupees for a decent place. Approx. 120 - 150 USD. Kandy would be same.
The problem is finding a room because they are much in demand and therefore getting a room for a reasonable price is a challenge, as the owners tend to rip off. Having said that, there are really good places too. The best thing is to stay at a tourist hostel a few days and find one from the next month, or get someone in Sri Lanka to find one for you.
But a few kilometers away from the major cities you can find cheaper homes. If you can be specific about the city I can recommend some area close to it.
Food ranges from 1 USD to 10 USD depending on what you eat. A normal Sri Lankan meal will be around 1USD at a normal shop but you must find a good, clean one which is a bit of a challenge.
Look at the climate, facilities, and generally the local culture of the areas before you decide solely on the price. Normally Colombo and Kandy are more desirable for a foreigner.
Hope this helps. I can answer if you can be specific about any area.
