Quick answer: Singapore runs about EUR 130 per day mid-range - noticeably less if you eat local and walk. Cheapest window: February-April - the driest months, with July also decent; expect heat and humidity year-round.
What things actually cost
hawker centre meal
4-6 EUR
kopi (local coffee)
1.30 EUR
MRT ride across town
1.40 EUR
beer at a hawker centre
5 EUR
Gardens by the Bay two-dome ticket
22 EUR
mid-range hotel double room
120-160 EUR
Eat well for little
Hainanese chicken rice at Maxwell Food Centre (Tian Tian) - the classic plate, about 4 EUR
Chilli crab at a seafood restaurant like Jumbo - messy and iconic, budget 40-55 EUR for a whole crab
Laksa at 328 Katong Laksa - coconut noodle soup with the noodles pre-cut, about 6 EUR
Kaya toast set with soft eggs and kopi at Ya Kun Kaya Toast - classic breakfast, about 4 EUR
Satay at Lau Pa Sat Satay Street - 10 skewers with peanut sauce for about 8 EUR, evenings only
Money mistakes to avoid
Eating only in Marina Bay restaurants - hawker centres serve better food at a tenth of the price
Forgetting MRT rules - eating, drinking or carrying durian on trains brings real fines
Underestimating the humidity - plan indoor stops (museums, malls) for midday and walk mornings and evenings
Paying cash for every transport ride - get an EZ-Link card or use a contactless bank card for fare caps
Transport without the tourist tax
From Changi Airport the MRT reaches the centre in 35-45 min for about 1.50 EUR, a taxi costs 20-28 EUR. In town the MRT plus occasional Grab rides covers everything; most MRT trips cost 1-1.50 EUR.