Sepentine is from the jade family and in carvings may be very expensive but much less expensive than fine jade or jadeite.
Serpentine is often sold as jade or jadeite. This is unethical. However some serpentine such as a bright greenish yellow are commonly called lemon jade in the industry and this practice is not unethical since the industry has used this name for decades and it is usually not an attempt to fool a buyer.
A good seller will usually let buyers who may not be gem savvy know that it is a cousin of jade and not jade or jadeite.