The cost of living in Bangladesh varies greatly depending on where you live (city and suburb, the size of apartment, whether you predominately buy local or imported food and other goods, and how often you eat out and where you eat out).
E.g., If you live in a reasonable 3 bedroom apartment in Gulshan or Baridhara which is where most expats live, eat out say once or twice a week at a mid level restaurant, buy 60% local food and 40% imported, and hire a house maid. Your average monthly living costs will be around $2,500 to $3,000 per month. Whereas if you chose to live in say Banani or Uttara, buy very few imported goods, and mostly ate out at local restaurants (beware of the hygiene and adulterated foods), and chose not to have a maid. You could live on about $1,200 per month