Approximately $20.99 - $22.99/month (depending on your region and commitment).
It is nearly half the price of the Single App plan. The Only "One-Time Buy" Exception Is Adobe Photoshop a desktop or web app? - Facebook
This is the most straightforward way to get "just" Photoshop, but it is actually than bundled options.
If you want to use the full version of Photoshop, you have two primary "standalone" paths, though both require recurring payments: 1. Photoshop Single App Plan