Almost nothing but that's because I'm just starting out. However, I do want to point out that you should focus on the total amount of money that you earn. The average amount per hub does not mean very much because only some of your hubs will become really good earners.
It's really difficult to make every hub earn very well but the more hubs you have (or the better each hub is), the more you'll earn. There are people who earn $1000+ dollar per month while others only make $100 or far less per month.