Welcomer to the forum C_Trout
There are several ways to determine which specific cut square /envelope you have. They have a heavy emphasis on which die was used. The US Postal Stationary Society has probably the most comprehensive website to identify your piece. This is their website
https://www.upss.org/As far as values go, I usually go to
https://www.stevelevinestamps-plus....-stationery/ to see what they are currently going for.
I hope this helps you on your quest. Good Luck