I'm looking for a source on values for older Eley and Kynoch nitro express ammo (single cartridges). Specifically .450 through .600 calibers. Thanks.

Join the International Ammunition Association (IAA). they offer a News letter/magazine called the "Journal" every 2 months with ammunition articles. Each issue has a section on cartridge values. Foe example the March 2024 issue covers "Inch Pistol & Revolver Cartridges". The next issue might cover "Metric Pistol & Revolver Cartridges". One of the upcoming issues will have a section that covers "British Sporting Cartridges". An E-Journal membership cost $25 (U.S.). I believe you can even purchase back issues once you are a member. Visit their website at cartridgecollectors.org.

Thanks!

Also:

https://cartridgecollector.net/