How much money to test Bing

The best answer, in my opinion, is the more you spend the better the test.

You have to purchase enough data to be useful. $50 - $100 isn't going to get you very much useful data. The results you see on such a small amount are very likely random. Meaning there isn't much chance that the next $100 you spend will have the same results.

How much you need to spend varies a great deal on how many keywords you are targeting and the average CPC of those selected keywords. Also the average deviation from mean, will influence how much data you need to collect to get actionable data.

So, in some cases $100 will give you a little bit of actionable data, and in other situations you might need to spend several thousand dollars to reach the same confidence level. Way too many variables to throw out a figure that works for every situation, unless you are throwing out a very high figure as a recommendation for all situations.

You should plan to spend at least $1000, to ensure that you are getting enough actionable data, but try to limit the scale and scope of your campaign to ensure you are gathering enough data to make reliable conclusions. It might take less, it might not. You can always stop your test early if you have enough data to merit an earlier conclusion, but it won't go wasted if you allow for a larger ad spend to get reliable data.

You don't want to make a bad decision, based on invalid data, that could have easily been avoided with a proper test.