Hi Caitlyn!
The difference in price largely comes down to the age. Amber is polymerized over a much greater period of time than copal, so there is added historical value. In terms of distinguishing the two - amber is harder than copal resin, so you can use a mohs test kit to verify that you have amber which should be about 2.5 to 3 mohs, or copal which is about 1.5-2.5. You also can tell which you have under a UV light - amber will shift to a blue color, while copal will display little to no change.
