OBDII (DTC) error codes

Can anyone familiar with OBDII or DTC codes point me in the direction
of the likely cause of the following error code???

P0411 - Secondary Air Injection System Incorrect Flow Detected

Obviously VW or any of their dealers treat such information as a
big secret...

The VW Service manual (from Bentley publishing)
does not even mention any secondary air injector.

It sets when the ECM senses the secondary air injection pump is functioning, but the quantity of air is found insufficient by the O2 sensor. It could be that the pump is not puming air, the SAI solenoid valve is not functioning, the vacuum or pressure hoses are defective or the combination valve isn't functioning. You should probably verify the O2 sensor is accurate also. Year & engine size would be helpful.

Sorry!
It's a 1999 (MK3) Golf GTI VR6 (2.8L).
There have been no mods done to the car
or engine whatsoever.

It has been giving that code for over 4 months.
There has been little noticeable difference in the engines
performance.

Let's say it IS the secondary air injector/pump...
What is the (likely) cost of such a part???

Haven't done one myself. It's a dealer only item anyway, so give them a call.