Quick answer: Cancun runs about EUR 90 per day mid-range - noticeably less if you eat local and walk. Cheapest window: December-April - dry season with warm days and low humidity; avoid September-October, the peak of hurricane season.
What things actually cost
taco al pastor
1 EUR
local R1 bus ride
0.65 EUR
round-trip ferry to Isla Mujeres
22 EUR
Chichen Itza entry
30 EUR
beer at a beach bar
3 EUR
cenote entry
10 EUR
Eat well for little
Tacos al pastor at Taqueria Los Chachalacos or any downtown taqueria - about 1 EUR per taco
Ceviche at Mercado 28 - big fresh portion for around 8 EUR
Cochinita pibil (slow-roasted pork) at La Parrilla - about 10 EUR
Marquesitas (crispy crepe with Nutella or cheese) from street carts on Parque de las Palapas - about 2 EUR
Fish tacos at El Fish Fritanga in the Hotel Zone - about 6 EUR for a plate
Money mistakes to avoid
Staying only in the Hotel Zone and paying double for everything - eat downtown at least a few times
Getting into a taxi without agreeing the price first - fares are not metered and tourists get quoted 3x
Arriving at Chichen Itza at midday - go at 8 am opening to beat the heat and the tour buses
Exchanging money at the airport - rates are poor; use bank ATMs in town and pay in pesos, not dollars
Transport without the tourist tax
From the airport, the ADO bus to downtown costs about 5 EUR; pre-booked shuttles to the Hotel Zone run 15-25 EUR. In town, the R1/R2 buses run the whole Hotel Zone strip for about 0.65 EUR per ride.