What is the value of an Eastman ETR 520 brass trumpet?

There's no way to tell you how much your Trumpet is worth without seeing it in person... The value of a trumpet doesn't just depend on what brand it is. It depends on age, condition, playability, among other things. If you contact a reputable Instrument Repair shop in your area, they can likely either tell you how much it would be worth, or give you a rough estimate.