Power bank price in Bangladesh

How much is the Power bank price in Bangladesh? I saw many names and branded Power banks selling online and in the store but where I can get the exact and accurate price for the Power Banks? Also, what is the difference between one model to another model how do I will know which one is the best power bank?
How will I know which capacity of the power bank is OK for my mobile phone? How many times I can charge from one power bank? I know I already asked many questions but actually, I really need to know more about the Power Bank price in Bangladesh before I buy it. any help on this matter?