Duplex. You can still find new, old stock duplex out there. They sit in the shelf for $1000 but sell for $700-$800 in my area (TN/KY). Mildot used should go about the same. I’d expect around $800-$850 depending on condition and how long you’re willing to wait for a buyer. Original 700/300 builders need this scope and a lot of 50 cal shooters liked the Ultra. 140 MOA internal elevation!
If it was modified by Premier Reticle, you’ll probably have to practically give it away. They did excellent work when Dick was working these scopes but unless something changed, Leupold will not warranty them anymore.