I recently purchased a cheap usb-ethernet adapter for my Macbook Air for just 5 EUR.
The model name is KY-LANRD9700, it’s a usb 2.0 to ethernet 100Mbps adapter and it uses the AX88772 chipset.
The USB id is: 0x9700:0x0fe6
This chipset is fully supported under MacOSX. You can download the driver from ASIX website.
No need to spend 30 EUR on the official usb-ethernet adapter from Apple!