How much should it cost to replace 2009 BMW 328i serpentine belt?

That's not the way it works. You go to a place like this and just order one

https://shop.advanceautoparts.com/find/bmw-328i-starter-relay

If you mean the location in the box?

https://www.google.com/search?q=bmw+328i+2009+relay+diagram

..

https://www.ebay.com/itm/2009-2013-BMW-328i-E93-LCI-OEM-LEFT-DRIVER-DOOR-LOCK-LATCH-ACTUATOR/233095607815?fits=Year%3A2013%7CModel%3A328i%7CMake%3ABMW&hash=item364594b607:g:ggoAAOSwfMlcPfdn:sc:USPSPriority!73533!US!-1

I, feel sorry for you, after reading your problem, but don't worry you visit the Reliable-Store they will help you for sure.
Best Manuals

Try this - http://www.wikihow.com/Pair-Your-Bluetooth-Phone-With-a-2006-2009-Bmw-3-Series

I assume that the squeaky noise stops after 15 minutes, not the engine. If that is the case, you may have a very worn belt. This won't cost you a ton of money. You have a serpentine belt on your car, so that is a likely culprit. You may also have a bad battery, which in cold weather would perhaps have just enough to start the car but would also demand the alternator (and by extension, the belt) to provide enough amps to recharge the battery after a severe draw,
Before the winter, I recommend a new serpentine belt and a new battery, after a test of your alternator output and the battery at a good shop. You should not expect to spend more than $200 max, if everything is wrong.

not real likely unless battery is goin dead and alt.is bad.Chech the small ground cable attached close to battery on the frame.i have had this problem recently and seen it happen alot.its a small black wire and usually their are sometimes other grounds to body at rear of engine.any of these can make guages act up then stop and headlights go out or all lights and speedometer stop.all kinds of weird things,dim interior light,dash light so on,hope you fet it.

RealOEM.com

With the hood open and the engine running, have someone turn the wheel so you can spot the power steering gear box. There will be two lines from that box leading up to the power steering pump. That pump is driven by the serpentine belt. Atop that pump there will be the filler cap you are looking for.

Hope that helps.

ash

the gas tank pump is malfuctioning or the check valve on the evpaerative canister purge valve is malfunctioning.

This just MIGHT help. I hope so.
German cars usually have one item on the route of the serpentine belt that is adjustable.
Traditionally, the way this is adjusted is with a long threaded shaft like a screw, and on that shaft is a lock device to lock the adjustment of the adjusted accessory.
Many times, it is the alternator that is adjustable.
You would loosen the locking clamp, then turn the nuts on the threaded rod, and as you move the part (alternator, for example) towards the engine, the belt will loosen.
Replace the belt and then tighten the nuts and tighten the lock.
God bless your efforts.