So … that box is run by a volunteer sysadmin who doesn't charge you? … and doesn't mind getting up at 3am to replace a drive?
That server has perfectly reliable power and environmental setup so you never have prolonged downtime or a double disk failure?
You're okay losing everything if someone makes a mistake running that server since backups cost too much?
You have higher-level software which tells you when data on that RAID array is corrupted? Your free sysadmin periodically runs an audit to make sure that the data stored on disk is what you originally stored? That's what I was referring to with scrubbing: even with RAID corruption happens and most storage admins have stories about the time they found out it'd happened after the only good disk failed, been written to tape, etc. The best solution is to actively scan every copy and verify it against the stored hashes for what you originally stored, which also protects against cases where a bug or human error meant that e.g. your RAID array faithfully protected a truncated file because the original write failed and nobody noticed in time. S3 provides a strong guarantee that you will get back the original data you stored or an error but never a corrupt copy and that you can prevent storing a partial or corrupted upload. If you roll your own, you need to provide those same protections for the full stack or accept a higher level of risk and perhaps mitigate it in other ways (e.g. Git-style distributed full copies with integrity checks).
Again, I'm not saying that it's impossible to pay less than S3 but your response is a bingo card for the corners people cut until something breaks and they learn the hard way why raw storage costs less than a supported storage service. Doing this for real adds support cost for the OS, your software, security, monitoring, backups & other DR planning, etc. If you use S3, Google, etc. you get all of that built into a price which is known in advance, which is a significant draw for anyone who wants to spend their mental capacity on other issues.
Many places don't have enough storage demand for that overhead to pay off in less than years and startups in particular should be extremely careful about spending their limited staff time on commodity functions rather than something which furthers their actual business. If you're Dropbox, sure, invest in a capable storage team because that's a core function but if your business is different it's time to look long and close at whether it makes any sense to devote staff time to saving a few grand a year.