Nowadays it's popular to say that people worship money, and perhaps that is true in some cases. But in the Bible, and still in many places today, people worship idols or various shapes and forms because they are thought to represent a deity or "power" of some kind. It is not a matter of what idol or image is made of. The golden calf is one specific example of an idol. Yes, it was made of something very valuable, but they people were not worshipping the gold. The fact that the calf was made of gold shows them giving a valuable offering to the god represented by the calf. They were certainly hoping/praying for power and/or protection from this god, and the calf was a physical representation of this god.