I get asked this question virtually everyday by authors who never self-published, therefore they have no idea how many books are selling each day until they receive their report months down the road. I'm by no means an expert, but after being an Indie Publisher for a couple of years, I've had books ranked all over the place and once in the Top 100 in the entire store. There's always lot's of variables that can change these numbers like time of year, but basically you can gauge your daily sales based on the following rankings for your kindle book. Paperbacks are totally different.
Ranking Daily Sales
101 to 400 2500
Top 100 3000 may get you in the #100