It would be handy to know what type of files we are talking about here.
I am a reseller, so have a lot of disk space and bandwidth available but one of my hobby sites which is Land Rover related (I know, really should get out more) has downloadable pdfs of the parts manuals and also .iso's of the online documentation systems and these are big and very very popular.
The site, html, graphics etc is hosted on my server but the pdf's and iso files are all stored on S3 as I think the hit would be too much for my server and impact on my customers sites.
Ok, this means I have to pay for the S3 but thankfully I get enough donations to cover the costs so I'm square which is great.
Sorry, this isn't really an answer to your question, more my personal experience.