this link has a few ideas but the main point is " a working foreign national having work permit have to pay 25% income tax, which was raised at 30% for the current fiscal, came into effect from July 1" and since its 2017, i suppose the tax can be higher now.
As for the cost of living, 30,000 BDT is fair enough for a standard living but 50,000 is what gives you a good living condition with a good rented flat/place, food, clothes, even going out once in a while. If you take buses, you can save a lot but will tire you out cause our public transporation is horrible. And transportations like CNGs cabs can empty your wallet...so 50,000 is good
