It depends on the number of days that you are sojourning in Bucharest.
My first suggestion is to book a room at the hotel with a minimum of 1 month, this way you’ll spend less on the hotel room.
If you are planning to stay 3 nights, then you should consider the cost of public transportation, food, and fun.
The first thing that you are going to spend money on is the trip from the airport to your hotel. If your hotel is in the center of Bucharest, then you’ll spend on taxi something between 10 and 15 euros. This goes also for the return. So, keep in mind to have 30 euros for these 2 trips alone: from the airport to your hotel and from the hotel to the airport.
Concerning the food, keep in mind that dinner and lunch meals are anywhere between 5 and 15 euros.
Breakfast is anywhere between 3 and 7 euros – when not taking the breakfast at the hotel.
Coffees are something between 1 and 3 euros. I don’t suggest you go to Starbucks even though Starbucks is super famous all around the world. I recommend you try the coffee from 5-to-go coffees. This brand is a Romanian brand and all their coffees are 5 lei (1 euro).
Snacks are something between 0.50 Eurocents and 2 euros.
Thus, per person, you should consider the fact that you can spend between 10 and 25 euros for food.
Regarding public transportation, metro tickets are 2,5 lei per 1 round-trip, and bus tickets are 0.30 Eurocents. I recommend you take only the metro and eventually Uber or Bolt if you need to move from one place to another. Buses are very crowded, uncomfortable and sometimes confusing.
Thus, per person, concerning public transportation, consider spending between 0.50 Eurocents and 10 euros (if you take the Uber) per day.
Museums are not expansive. Museum tickets are between 3 euros and 10 euros.
In conclusion, spending 50 euros or less per day is something that you should expect.
Comparing to other European capitals, Bucharest is not so expansive and you can do a lot of things with few Eurocents.