When Dad and I finally got around to building his "Full Basement Layout", we bought the lumber, plywood and homasote from Menards, but almost everything else was secondhand from somewhere.
We started with his 1950's childhood 4 x 10 layout, added on a bunch of used Gargraves track, bought up various Ross switches and crossovers from auctions that we'd keep eyes on, and gradually filled out the layout just as Dad wanted it. Probably paid about 60% for the layout and table versus buying everything new.
For the rolling stock...we had dad's old conventional stuff, then we had around 30 cars of rolling stock that we purchased 20+ years ago at an in-person auction for about 40% of retail, and then gradually filled out the rolling stock with other auctions and used things we found online.
For the motive power, I can honestly say we've NEVER bought a new unit retail. Every single piece we have was either one of dad's original conventional engines or an engine we bought as "not working" from online or the MTH liquidation auction. I learned over 5+ years to repair these to working order then eventually to convert to PS2 or PS3 after we bought a used TIU from this forum.
We would never have been able to afford the extensive collection of locos and rolling stock that we have. I'd say we paid maybe 30-40% of retail for all our equipment (and in the case of the 8 MTH big steam premier engines, maybe 10% retail), but the trade off was MANY hours of tinkering and soldering and prying and buying tools and cutting my fingertips and ruining PS2 boards.
But those were hours I spent with my dad (and sometimes my son, too) and I say I came out way on top for the time we got before he passed.