If you're willing to put in a little work, Seafile fits the bill. For less than 5 users, you can access the "professional" version for free. I run it on a chea

If you're willing to put in a little work, Seafile fits the bill. For less than 5 users, you can access the "professional" version for free. I run it on a cheap Azure VM, backed with with Azure blob storage. It's $15/m for "cool" tier storage (not too far off your target), or $1/m for the "archive" tier. Unlike Dropbox etc, you can also sync files from anywhere on any disk - you're not limited to a single "Dropbox" folder. You get a nice web UI that let's you upload/download from there if you want, and you can also stuff with others if you want.