Quick answer: Paris runs about EUR 150 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April-June and September-October - pleasant walking weather and lighter museum lines than July-August.
What things actually cost
espresso at a cafe counter
2.50 EUR
croissant
1.50 EUR
metro single ticket
2.50 EUR
Louvre entry
22 EUR
bistro lunch menu (2 courses)
20 EUR
glass of wine at a bar
6 EUR
Eat well for little
Croissant from an award-listed boulangerie - about 1.50 EUR, best before 9am
Jambon-beurre baguette sandwich - the classic 6 EUR lunch
Falafel on Rue des Rosiers in Le Marais - about 9 EUR, expect a line at L'As du Fallafel
Steak frites at a neighborhood bistro - 18-25 EUR
Crepe from a street stand - 5-7 EUR with nutella or ham and cheese
Money mistakes to avoid
Not booking the Louvre and Eiffel Tower online in advance - walk-up lines waste half a day in peak season
Eating at cafes directly on major squares like near the Champs-Elysees - a coffee can cost double what it does one street back
Trying to see more than two museums in a day - the Louvre alone is exhausting; pick wings, not the whole building
Only using taxis - the metro is faster for most cross-town trips and a single ticket costs about 2.50 EUR
Transport without the tourist tax
From Charles de Gaulle, the RER B train reaches central Paris in about 35 minutes for around 13 EUR. In town, walk plus the metro covers everything; a single ticket is about 2.50 EUR and contactless works at the gates.