In addition to what tdracer already highlighted, with regards to PMA parts.
This article may be of some interest.
http://www.chromalloy.com/files/news...542415a369.pdf
Dated December 2009
"There are now more than 300,00 approved PMA parts, and the number grows each year by about 35,000"
"To give an example of how our prices compare to OEMs, you can pay $50-83 for a wheel seal, while our prices are $30-50. This adds up to a large saving for an airline using 500 seals each year"