VeraCrypt

VeraCrypt exists for one situation: you have files that must be encrypted, and the drive they live on has to be readable somewhere other than a Mac. Make a container of whatever size, mount it with your password, and it behaves like a disk in the Finder until you dismount it. Plug that drive into a Windows machine, install VeraCrypt there, and the same volume opens. That portability is the whole reason to bother.

There is no purchase to think about. The project is a continuation of TrueCrypt maintained by IDRIX, the source is public, and releases still arrive with real security fixes rather than cosmetic ones.

The cost is inconvenience rather than money. macOS support rides on macFUSE, so you approve a system extension and occasionally wait after a macOS upgrade before you can mount anything. The app looks like the security tool it is. And the honest advice for most people is that you do not need it: if you only want your own Mac's disk encrypted, turn on FileVault and stop, because it is already there and needs no extensions. Reach for VeraCrypt when the encrypted thing has to travel between operating systems, or when you want containers you can copy around like files.