Quick answer: Osaka runs about EUR 115 per day mid-range - noticeably less if you eat local and walk. Cheapest window: March-May for cherry blossoms and mild weather, October-November for autumn color - avoid the humid rainy season in June-July.
What things actually cost
metro single ride
1.20 EUR
takoyaki (8 pieces)
4 EUR
konbini onigiri rice ball
0.90 EUR
ramen bowl
6.50 EUR
Osaka Castle main tower entry
3.60 EUR
draft beer at an izakaya
2.50-3.50 EUR
Eat well for little
Takoyaki - octopus batter balls from a street stand like Wanaka or Kukuru, about 3.50-4.50 EUR for 8
Okonomiyaki - the savory cabbage pancake, queue at Mizuno in Dotonbori, about 8-11 EUR
Kushikatsu - deep-fried skewers at Daruma in Shinsekai, a set runs about 10-15 EUR, never double-dip the shared sauce
Kaiten (conveyor) sushi - plates from about 0.80-1.50 EUR at chains like Kura or Daiki Suisan
Ramen or udon lunch - a solid bowl almost anywhere for 6-8 EUR, pay at the ticket machine first
Money mistakes to avoid
Eating only on the main Dotonbori strip - one street back (Ura-Namba, Hozenji Yokocho) is cheaper and better, and many famous stands have shorter-queue branches
Buying a nationwide JR Pass for a Kansai-only trip - it rarely pays off, use an ICOCA card plus point-to-point tickets or a Kansai area pass instead
Double-dipping kushikatsu skewers in the communal sauce - it is the one food rule Osakans genuinely enforce, use the free cabbage to scoop extra sauce
Cramming Osaka into a day trip from Kyoto - the city comes alive at night, sleep here at least two nights for the evening food scene
Transport without the tourist tax
From Kansai International Airport, the Nankai rapi:t or JR Kansai Airport Express reaches the center in 40-50 minutes for about 7-9 EUR. In town, get a rechargeable ICOCA card and use the metro at about 1.20-1.80 EUR per ride - an all-day Osaka Metro pass is around 5 EUR.