I set daily limits/campaign. Monthly depends on how many campaigns. Last month was ~2400 = 80/day. You have to spend money to gather data. You can't split test and tweak campaigns without data.
My mentor told me to start at a daily limit of 25$... and I think he was right. I did set every campaign lower at 20 a day. So if your question is a monthly one.. I guess it would depend on how much you set your daily limit and how many campaign you are going to run or currently running.
A month before i was spending like: 400USD a week, but from last moth, my cpc reduced due to good score, so from last month its like: USd 225.
Spending is irrelevant. What's important is ROI. If you have an awesome ROI, you could spend millions per month, knowing that you're going to see an positive return. Learn ROI and the rest is cake.