Not much. This kind of small cheap imported single-action .25 caliber automatic is often called a "Saturday Night Special." I'd guess the value is about $75 if it works and is in fair to good condition.
25-60 USD or so
50 dollars
25-100
10-100 USD or so
You did not say which model. The Erma-Werke KGP68, which looks like the traditional "Luger" pistol, is about $250-$500, depending on condition. If you have a different model, please repost with that information.
25-125 usd
I know of no handgun that has a one inch diameter bore.
25 USD
Best left to a gunsmith.
The cartridge is common, and is typically referred to as the .25 ACP (Automatic Colt Pistol). In European markets, it's more commonly known as the 6.35x16SR.
25-80
25-160