Is it safe to drink the tap water?

No. Use the refill machines or buy large 5L jugs from 7-Eleven to keep costs low.

Are night markets cheaper for food?

Sometimes, but they are often tourist-oriented. Look for stalls where local office workers are eating during lunch hour for the best prices.

Bangkok on a Budget: How to Make $30 Last All Day

You can comfortably explore Bangkok on $30 USD per day by sticking to street food, utilizing public river boats and the Skytrain, and prioritizing free temple sites. Avoid taxis and Western-style restaurants to keep your daily spending in check.

Master the transport network. Avoid taxis and Tuk-Tuks, which charge premium 'tourist' rates. Use the MRT (subway) and BTS (Skytrain) for city travel, and the Chao Phraya Express Boat for river transit, which costs as little as 16 THB ($0.45 USD).
Eat the street. Eat exclusively at street stalls or food courts in malls like Terminal 21. A full meal of Pad Thai or boat noodles at a stall typically costs 50–70 THB ($1.40–$2.00 USD).
Choose your temples wisely. The Grand Palace costs 500 THB ($14 USD), which is a budget-buster. Visit Wat Saket (100 THB) or explore smaller free neighborhood temples to save money while still getting your cultural fill.
Use public water dispensers. Stop buying individual plastic bottles of water. Use the public water refill machines found on many street corners; they charge 1 THB per liter, saving you significant money over a week.

Is it safe to drink the tap water?: No. Use the refill machines or buy large 5L jugs from 7-Eleven to keep costs low.
Are night markets cheaper for food?: Sometimes, but they are often tourist-oriented. Look for stalls where local office workers are eating during lunch hour for the best prices.