The symptoms the repair page lists
The VANOS solenoid page lists rough idle and stalling at stops, hesitation or a loss of power at highway speed, VANOS codes such as 2A82 and 2A87, and a rattle on acceleration that owners call VANOS rattle. The cars named are the 128i, 328i and 528i and the X3 3.0si on the N52, and the 135i, 335i and 535i on the N54. See VANOS Solenoids, BMW N52 engine and BMW N54 engine.
Swap the solenoids before buying parts
The page says the two solenoids are interchangeable top and bottom, and that swapping them is a quick way to see whether a fault moves from intake to exhaust before buying parts. That is the diagnostic step in our data. We do not have a sourced cleaning procedure or a price for cleaning them, so this guide does not describe one.
What replacement involves
An owner puts the job at about 15 to 20 minutes for both solenoids. The parts list says to change the oil right after. The page is still thin on US independent shop prices: the priced reports are dealer quotes and DIY parts. UK prices in pounds are noted on the repair page and are not converted or counted.
A rough idle has other causes. Plugs, coils, injectors and carbon are covered in the BMW Rough Idle and Misfire: Causes and Repair Costs. If the rattle is on a cold start rather than on acceleration, see the BMW Cold Start Rattle, Whine and Squeal: Causes by Engine.
Where this shows up on a used car
On an E90 328i the engine is the N52, and the valve cover and oil filter housing are the other priced jobs on that car. Oil on the belt from that housing is the BMW Oil and Coolant Mixing at the Filter Housing. The model page is BMW E90 repair costs. On an N54 car the fuel system is the larger repair; the Why BMW N54 Injectors Cost So Much explains why those parts dominate a quote. Intake-valve carbon on those cars is the BMW Walnut Blasting and Carbon Cleaning. Smoke and oil in the spark plug wells are the built-in crankcase vent, in the BMW PCV and Crankcase Vent Symptoms.