To correct my errors in my previous post. I was at work and provided you numbers from Memory.
These are the BMW OEM P/N's and price I paid in $USD at BMW of San Diego
13-72-1-744-869 Air Filter $21.74 + Tax
64-31-1-000-004 Microfilter $29.90 + Tax
You can go to Circle BMW and enter the above P/N's to determine the MSRP for these parts. If you do this you will find the Air Filter is $18 and the Microfilter is $26. http://www.circlebmw.com/parts/price.htm
Order these parts from Circle BMW or Pacific BMW. You may get them cheaper but you will have shipping charges.