Currency in Sri Lanka is Rupees (LKR). You can easily exchange money as you land in Sri Lanka from the exchange counters located in the Arrival hall right after Customs. All banks, currency exchange counters, and some hotels will change major currencies like the US Dollar, UK Sterling Pounds, and Euros. Even though you can change many currencies it's always better to bring US dollars as its one of the easiest currencies to change in Sri Lanka. Here's a link to get the latest updates on exchange rates https://www.xe.com/currencyconverter/
Master/Visa credit cards are also accepted in the midrange and top hotels in Sri Lanka.
Rupee coins come in denominations of one, two, five and 10 rupees. Notes come in denominations of 20, 50, 100, 500, 1000 and 5000 rupees.
Approximate cost per day
Budget - Budget hotel, food, and local transport will cost you around LKR 5000-7000.
Mid - A simple 3-star hotel, food, and rented vehicle will cost you around LKR 6000 - 18000.
Luxury - 4 to 5-star hotel, food and rented vehicle with a driver will cost you more than LKR 18000.