The main reason is usually market volume, not a huge image-quality difference. SD cards are produced and sold in much higher quantities, so they benefit more from economies of scale. CF was once more common, and when that was true the price relationship could be different.

CF and SD also differ in design. CF cards include more of the controller hardware in the card itself, while SD relies more on the reader/camera side. That can affect cost. CF cards are also physically larger and generally sturdier, which some photographers value if they carry and swap cards often.

In some setups, CF can offer higher performance, but only if both the camera and card reader are fast enough to take advantage of it. For many users, that speed advantage is small or irrelevant.

So the practical answer is: CF cards may cost more because they’re made and sold in lower volumes and use a different, more robust design. Their main advantages are durability and potentially higher top-end speed, but for most buyers the best card is simply the format their camera supports.