coins and medals are worth much less than you think. Some coins are not made of precious metal, but of material that does not cost much in itself.
Value of circulation coins depends on supply and demand: If many enthusiasts have an interest in it, but there is little to buy, you can make a significant profit. The spectrum ranges from "beautiful" with clear signs of wear to the flawless "uncirculated". The difference in value between them can be significant, so you should never clean your rare coin yourself.