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
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.
Best left to a gunsmith.
25-80
25-160