All of the above.
As someone else mentioned, if he is part of a LEGO user group, he probably gets some bulk items at an unknown discount (program allows user groups a bulk purchase on certain elements once a year).
Not sure if he collects minifigs, but buying sets on sale or clearance and selling the minifigs is an inexpensive way to accumulate pieces. Only issue is you get stuck with a LOT of duplicate pieces you don't really need.
Pick-A-Brick wall at LEGO stores is the cheapest way to get lots of parts for the average person, but there is a very limited selection and it varies over time.
You aren't done yet so I will buy all 662 and sell them to you at a higher price when you need them. Are you making lifesize wings for some steampunk cosplay?
When you see what I am doing with them, it will blow your mind. Not Easter island tier, but that sort of blow you away reason.