Bad PCV could cause these symptoms, since unmeasured air enters into the intake.
PCV (Crankcase Vent Valve) could be tested by a manometer.
It's from the BMW repair manual:
A properly functioning pressure control valve is designed to maintain a slight vacuum (approximately 10 - 15 mbar ) in the crankcase which assures reliable crankcase venting during all engine operating conditions.
A malfunctioning crankcase ventilation valve may cause the following complaints:
* Engine runs rough
* Whistling noise from crankcase ventilation valve
* Check engine light on - possible DME faults stored: misfire all cylinders, oxygen sensor/mixture faults, etc.
You can do this:
While the motor is running at normal operating temperatures, loosen up the oil filler cap and try to lift it up. If there is a strong vacuum sucking the cap back, the PCV valve is bad.
I don't know if their estimate includes all the vent hoses, I'd strongly suggest to replace those also.
Zee