The first way the gilded half-dollar is symbolic is that it appears to be real and impressive, like the flashy Otis D. Slemmons.
If you took false gold for real, it could ruin you, just as it nearly ruined the couple when Missie May accepted Slemmons.
Joe tries to make his body look like Slemmons', because he's taken the appearance for real too.
Slemmons didn't allow anyone to touch his gold because he knew it was fake, just as he was fake (and couldn't stay around after being touched).
Underneath was a fifty cent piece, which wasn't really gold—but which still had value, like Joe and Missie May's life.