Hundreds or thousands of dollars. It depends how big it is.
Of course, "gold crown" could mean two things. A crown is royal headgear, but there was a 5-shilling coin minted starting in the 16th Century called a crown. The market value of any old coin will depend not only on the original value but also the condition of the sample and how many of them are still around.
"Crown" can also refer to dental work, which can have a decent scrap value as well.