He was probably including a few "big" purchases like locos and such in addition to his $20 a week "background cost" from materials, etc. Still, whether $8,000 or $15,000, his point is valid - over a period of many years you don't notice it an ymore than the cost of getting that triple venti latte' every day at Starbucks.
What is great about this hobby (and many other hobbies, too, not just toy trains) is that you can enjoy it well on just about any budget. I think the key is to pick a budget that is not intrusive on other aspects of your life and that fits.
That said, there is a conflict in my life as to "budget." I'm fortunate to have a large monetary budget for toy trains right now - essentially I spend whatever I want - but I have a very limited time budget, because I'm working four days a week/ten hours a day to earn all that money I find myself wishing I had more time for the trains, as many here clearly do, even though I know working less would mean less money.
Back to the cost of rolling stock for a moment: I recently bought six RMT Coast Guard depressed-center flat cars with missiles for, I think, $19 per pair, or something ridiculously low like that (maybe it was $19 per car, not pair?). Can't remember), and also about that time a set of five Sherman tank MTH flatcars for around $65-$75 each. The two sets of cars were each worth what they cost - the RMT were very basic and small, and the MTH nicely detailed big cars with detailed tanks on each, and lots of added parts and details underneath, etc., and very, very low friction trucks -they roll downhill a lot farther than the RMT.