How much your trombone is worth will depend on the brand, the age and the condition of the trombone. To find it's general value, search for like items for sale on eBay. Be sure the age and condition is similar to yours.
3 Additional Answers
The value of your trombone will very significantly depending on the condition it is in, the brand, its age and how rare it is. Typically they value anywhere between $100 and $1,000.
If you are interested in learning how much your trombone is worth take a look at it and figure out the maker. Next look online at different sites that will give you a clue.
How much your trombone is worth depends greatly on the type of trombone and its condition. In my experience, the only way to truly find the value of your trombone is to have an expert appraise the instrument.