It varies. The general consensus is that an independent artist that owns their music and publishing will make roughly 60 to 70 cents on a 99 cent iTunes sale.
The exact number depends on the agreement with the distributor, whether or not the artist works with an independent label or publisher, shares in the songwriting credit, or owes royalties on the track (i.e. it’s a cover song). It is not possible for an artist to submit their content directly to iTunes.
Digital distribution to iTunes (and other digital platforms!) is low-cost and efficient, making it the ideal choice for independent artists who want to ensure that their music is available across many different channels, maximize reach and acquire new fans (and/or listeners).
Please also see the discussion on Quora.