Buy once instead of Dropbox
2 apps in the directory cover what people use Dropbox for. 1 of them is a one-time purchase, and one costs nothing at all.
Read the catch on each one before you switch — a one-time price is not automatically the better deal, and we say so where it is not.
The replacements
Syncthing
peer-to-peer folder sync with no cloud, no account and no quota
Free Open source
No official Mac app — you configure it in a browser, and the menu bar wrappers are third-party
ChronoSync
defined, repeatable folder sync jobs with free major upgrades
$49.99 Perpetual
Syncing to another Mac properly wants ChronoAgent, a separate $19.99 purchase