I have been a member of Priority Pass for a number of years now, and despite travelling Business or First for the majority of my trips, have still found several opportunities to get value from my Pass, especially in the US.
Marriott Rewards allow you to use points to get membership of this scheme for between 20k and 85k points, dependent on the level of membership.
I don’t think the 85k redemption is good value, but for the odd visit, the 20k redemption is good value:
Priority Pass — Priority Pass – Airport Lounge Access
Priority Pass – Standard Membership 20,000 View
Priority Pass – Standard Plus Membership 55,000 View
Priority Pass – Prestige Membership 85,000 View