How to decide, should I make 50 points bounty or 100 points bounty?
If I am offering a bounty to attract attention to some question, I usually choose the lowest possible amount. There are several reasons why I do this.
One of them is that there is already quite a lot of bounties and the question might remain unnoticed despite the bounty. In such case I would lose my reputation without gaining anything, except a few more views of the question.
The more important reason is that every subsequent bounty must be at least double of the preceding one. (See this post on meta.SO or FAQ on bounties.) It is quite possible that I decide to put another bounty on the same question. (For example, if I get several good answers and I want to reward more than one of the anwerers. Or if I get no answer and I want to try another bounty in order to attract some potential answerers by offering a new bounty.) If the cycle of offering a bounty is repeated several times, the amount of points can rise to quite a substantial number. (Although, as the FAQ explain, the maximum is 500 reputation points.)
You should first try to find out why your question did not attract an answer. If the question is hard to understand or it seems that it would be next to impossible to give an answer that you would accept, then setting a bounty will not help.
However, if the problem is that the expected answer will take above average effort to write down properly, then setting a bounty will help and the bounty should be commensurate with the expected extra effort needed.