The idle speed control actuator is attached to the throttle body which is on the inlet manifold. In the UK the cost was approximately £350 but I also had to have it fitted by the dealer because it needed to be electronically set. The actual fitting seems very easy but the need for it to be set !??
The fault could also be intermittent but was more prominent when the car was at operating temoperature. The diagnostic at the dealer clearly indicated the actuator to be at fault in my case and since this was changed the problem has gone. The dealer said that the actuators are prone to go at aprox. 100k miles.
Hope this helps.