Quick answer: Nice runs about EUR 130 per day mid-range - noticeably less if you eat local and walk. Cheapest window: May-June and September-October - warm sea and sunshine without the July-August crowds and peak prices.
What things actually cost
espresso at a bar
2 EUR
socca portion
4 EUR
pan bagnat
6 EUR
tram or bus ticket
1.70 EUR
glass of rose at a cafe
5 EUR
beach club lounger for the day
20-25 EUR
Eat well for little
Socca (chickpea pancake) at Chez Pipo near the port - about 4 EUR a portion
Pan bagnat (tuna nicoise sandwich) from a bakery - about 6 EUR
Salade nicoise at an old-town bistro - about 14 EUR
Gelato at Fenocchio on Place Rossetti - dozens of flavors, about 3.50 EUR for two scoops
Pissaladiere (onion and anchovy tart) from a market stall - about 4 EUR
Money mistakes to avoid
Expecting sandy beaches - Nice's beaches are pebbles, bring water shoes or rent a lounger at a beach club
Eating at Promenade-facing tourist menus - walk two streets back into Vieux Nice or the port for better and cheaper food
Not validating your tram ticket before riding - inspectors are frequent and fines are around 50 EUR
Cramming Monaco, Eze and Villefranche into one day - pick two, the buses and viewpoints deserve unhurried time
Transport without the tourist tax
Tram Line 2 runs from Nice airport to the city center in about 30 min for 1.70 EUR. The center is walkable and trams and buses cover the rest at 1.70 EUR a ride - coastal trains to Villefranche, Monaco and Antibes cost only a few euros each way.