Download and install studio one prime. It's free and you have a news feed which details discounts as and when they happen.
The cheapest way to go is buying something (hardware) with studio one artist bundled in and go thru the upgrade route. Or you could find a user who has spare artist keys, buying them off a user and then upgrade that way. Sometimes you can get cheaper artist keys via Reverb. I doubt anyone would be selling the Pro version on Reverb or other second user selling platform.
People tend to leave Logic or Pro Tools and then get Studio one Pro as a replacement which means you don't see Studio One Pro keys on the second user market.