There are several items that contribute to the total cost:
Materials – cast limestone is more expensive than other cast stone.
Size – a larger fireplace is more expensive than a smaller one.
Coverage – a complete floor-to-ceiling design that includes an overmantel is more expensive than a mantel-only option.
Complexity – a more complex or ornate design is more expensive than a simpler classic design.
Customization – an installation that requires extensive custom modifications is more expensive than a standard design