I got two for about $20 USD apiece, I think? But that was quite a few years ago, and prices may have changed since then. There are definitely different options at different price levels; I think I got pretty barebones ones. They also work with modern Windows and Linux! (I presume Mac as well, but haven't tried.)

Yubikeys can also be used with an authentication app that can be used on non-phones, if you don't have a smartphone!