Your gun is worth between 500 to as much as 3000. The gun is actually worth more when you have (If you have) original box, paperwork, etc. Just the gun alone is about 500 bucks (If you can find someone to buy it) A gun store will buy it for around 300 (again depending upon how much original stuff you have with it) maybe more.
The value of a Colt Diamondback .22-long rifle could be anything between $800 and $1,000. Its exact value will depend on the extent of its handling wear and how fast you want to move it. You can always have it appraised at an antique's gun store.