I think it's a good question, and one a lot of people want to know the answer to. One thing I love about KB writer's cafe and being indie in general is the attitude of openness and sharing. Being open with each other creates a community of encouragement so I'm happy to share my experience with anyone who asks.
I sold 141 books in the first full month my first book was available. It was only 99 cents, so that was less than $50. I also think that was mostly friends and family buying the book. I was able to release my second book quickly and sold 1134 that second full month (Dec. 2010), but I also listed that book at 99 cents. That meant just under $400. I had similar sales the following month. So, basically, I think I made about $850 in the first three months, but that included sales of a second book. I wasn't keeping records back then, so I'm not sure how many on book one alone. Probably around the $500 mark.
It wasn't until I released my third book in late Feb. 2011 that I really started to see money that made a difference. I listed that book at $2.99 and sold 3703 books the month it released, then over 6000 the following month (March 2011). That was also the first month I started actually tracking my sales and income. I made just over $6000 that month, which was truly a dream come true. For me at the time, that was a level of success I hadn't expected to ever see.
I think I was very lucky to see such quick success. Of course, back then I was able to put the books out very quickly which really made a difference. It was also a different world 2 years ago. There is a lot more competition now, so don't feel discouraged if you don't see huge success in three or four months. It might take time, but as long as you continue to write, you'll see it grow and grow.
I actually listed out all of my sales numbers by month in a blog post a while back if you want to take a look.
http://sarracannon.com/2012/09/milestone-100000-sales-the-numbers/