Quick answer: Bangkok runs about EUR 55 per day mid-range - noticeably less if you eat local and walk. Cheapest window: November–February (cool, dry season).
What things actually cost
street pad thai
1.50 EUR
BTS Skytrain ride
1 EUR
big bottle of Chang beer
2 EUR
iced Thai coffee
1.50 EUR
Grand Palace entry
13 EUR
one-hour Thai massage
8 EUR
Eat well for little
Pad thai from a street stall - 1.50-2 EUR, look for a queue of locals rather than a laminated tourist menu
Boat noodles near Victory Monument - intense small bowls for about 0.60 EUR each, order several
Street food crawl on Yaowarat Road - grilled seafood, kuay jab pork soup and dim sum, 5-8 EUR fills you up
Mango sticky rice - about 2 EUR in season from fruit carts, an institution for good reason
Som tam (papaya salad) with grilled chicken - around 3 EUR at any Isaan street kitchen, ask for mild unless you mean it
Money mistakes to avoid
Believing anyone who says the Grand Palace is closed - it is the classic setup for a tuk-tuk gem-shop scam, walk on
Taking taxis that refuse the meter - insist on the meter or use Grab, especially from tourist areas
Sitting in road traffic at rush hour - the BTS, MRT and river boats skip it entirely for under 1.50 EUR
Showing up at temples in shorts or sleeveless tops - you will be refused entry or forced to rent a cover-up
Transport without the tourist tax
The Airport Rail Link runs from Suvarnabhumi to the center in about 30 minutes for 1.20 EUR. Around town use the BTS Skytrain and MRT (0.50-1.50 EUR per ride), Chao Phraya river boats for the Old City, and Grab for short hops at 2-3 EUR.