You never specified a make and model. You could be looking at anywhere from $30 - $150, all things dependent.
What is the value of a Davis Derringer pistol magnum 2 shot?
10-60 usd
10-40 USD
The Butler 22 caliber single shot pistol was made by Derringer. The Derringer company was founded by Henry Derringer. Derringer pistols are the smallest handguns available and were most commonly used by women.
10-50 USD or so
10-140 USD depending on specifics
I THINK your derringer may be a RECK. Value about $25-$50.
35-90 USD
There is no .25 Magnum. There IS a .22 magnum, made by Davis, and a .25 ACP. Value is about $50-$75.
20-80 usd
No published data
50 or so