I am considering creating an app for the first time. Since it will represent a significant time investment, I am wondering if it will get a decent return on my investment.

I am thinking of making the app free and running it with iAd. Therefore, Ive been trying to find statistics about how much money developers typically get, but Ive had no luck. COULD SOMEONE RUNNING iAd POST THEIR TYPICAL REVENUE? Thanks.
I now that depends on various factors but im really want an average.