Quick answer: Colombo runs about EUR 45 per day mid-range - noticeably less if you eat local and walk. Cheapest window: December-March - the west coast dry season; May-September brings the southwest monsoon.
What things actually cost
kottu roti
2 EUR
milk tea at a kade
0.40 EUR
PickMe tuk-tuk for 3 km
1 EUR
rice and curry lunch
2.50 EUR
train to Galle (2nd class)
2 EUR
local Lion lager at a bar
1.50 EUR
Eat well for little
Kottu roti - chopped flatbread stir-fry you will hear before you see, about 2 EUR at a local joint
Egg hoppers with sambol at Palmyrah or a street cart - about 1.50 EUR for a round
Rice and curry lunch (veg spread) at a local 'hotel' - about 2.50 EUR
Isso vade - shrimp-topped lentil fritters on Galle Face Green, about 0.50 EUR
Crab curry at Ministry of Crab in the Old Dutch Hospital - the famous splurge, 25-40 EUR per person
Money mistakes to avoid
Negotiating with waving tuk-tuk drivers - book metered rides through the PickMe app for a fraction of the price
Entering temples with shoulders or knees uncovered, or with shoes on - carry a sarong and expect to walk barefoot
Underestimating traffic - a 6 km hop can take 45 minutes at rush hour, so plan sights by cluster
Expecting a swimming beach downtown - the city shore is for strolling; go to Mount Lavinia for sand
Transport without the tourist tax
Bandaranaike International Airport is 35 km north; a prepaid taxi or ride-hail into the city costs 10-12 EUR and takes 45-75 minutes depending on traffic. In town, tuk-tuks booked via the PickMe app cost roughly 0.30 EUR per km.