Rough Budget

Generally speaking, Sri Lanka is a pretty affordable country to travel around in. Transportation and food are ridiculously cheap that often times, a short tuk-tuk ride from the bus stop to your hotel would cost you more than the 4-hour bus you took to get to a place.

A 7-hour train ride can set you back around $1.50 (2nd class tickets) taking you through some of the most scenic parts of the country. The main expense in Sri Lanka would be accommodations, although there are lots to choose from. A good budget would be $25-30 a day (with $30 being pretty generous already). Listed below is a rough breakdown of expenses. For transportation, Uber is widely used in the main cities and local transport apps like Fair Taxi and Pick Me are great to have.

Accommodation

Budget: $8-10 (dorm) $15-20 (private)
Mid-range: $25-$40
Splurge: $50++ 

Food (Typical Meal For One)

Street food: $1:50-2
Mid-range restaurant: $6-8
Gourmet meals: $10-$15
Beer from local stores: $1.75
Beer in restaurants and bars: $3-4

Transport

tuk-tuks: Depends on where you are but generally speaking, tuk-tuks should cost $0.30/KM but this almost never happens unless you are amazing at haggling.
Trains: $0.50-1:50
Buses: $0.75-2