It depends on how you distribute your music to iTunes. Some e-distributors take up to 50%. I use Tunecore who charge a delivery fee and a yearly storage fee only. This way you get to keep all of the money from iTunes, etc. without another party taking a cut. For example, we get around $7 for selling a $10 album after the iTunes fee. With an older service we no longer use (who I won't name) we would have received $3.50 or less. Unfortunately, buying a CD at a show usually doesn't go into the SoundScan database, therefor not contributing to the artist's sales numbers.
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Here's an infographic on the proportion of money that goes to the artist and label for different types of music sales:
I don't have the numbers, but I asked this same question of a longtime musician pal of mine, whose album I wanted to pay for to support his band. He suggested going to iTunes. Short of paying someone in person at a merchandise table at a show, that's what he recommended. I'm afraid that I don't know the exact math on the revenue split.
Small bands don't even make merch per se, but they usually sell actual CD's or even vinyl. (My friend mentioned above released a 12" with a download code inside the jacket sleeve for 320kbps MP3 files online, nice way to split the difference.)