Of course that depends on what kinds of assets you serve and how many hits you get, but to provide a ballpark, I had a small personal site with few media assets

Of course that depends on what kinds of assets you serve and how many hits you get, but to provide a ballpark, I had a small personal site with few media assets on S3, and it consistently cost me US$0.12 per month. I think once it cost me 14 cents and I thought, "Wow, I must've been popular last month!" I didn't run analytics so I can't say how many hits it got, but traffic was probably fairly average for a personal site.