I'm assuming the question refers to money spent on model train related things.
For me, it's hard to say as very little of what I have, per item, was ever all that costly. I have paid as much as about $150 for a structure, and around $175-200 each for the four Bachmann ten-wheelers I have. The rolling stock, I was smart enough to get those back when you could still get them for around the $20-ish range. I'd have had problems buying all the freight cars I now have if I'd waited to the current price ranges. The most I've paid for a car is about $100 for a AMS coach, and about $85 for a laser kit of a caboose.
Many of the structures I have are scratchbuilt, and a few of those I bought second-hand off eBay and did paint/detail work on a few of them for really good prices. Haven't seen any good deals like that in a while. Again, I picked the right times to go looking for those.
lumber was a few hundred bucks, as I bought good lumber and didn't scrimp or save there.
The funny thing is the one item on the entire layout I like the most, was a present from my Dad. My father, a wizard with casting and lathe work, heard me say I was thinking of building a scale 'Civil War' memorial, and he volunteered to make a scale cannon barrel. Soon, I got a tiny little package with a perfectly scaled 12-pounder 'Napoleon' barrel made from brass!
A great deal of stuff was bought off eBay, and a lesser amount at a hobby shop in Tacoma I like going to, all bought over a 2+ year period while I was planning what the layout would be like, getting a turnout or freight car pretty much every time I walked in the place. It really evened the cost out over time that way (I was smart enough o do all the detail/paint/decal/weathering work as I bought each car, so once the layout was ready for trains, I just had to pull them out of the boxes, mount brake wheels, and put them on the tracks ready to go).
The key here is that I only bought stuff I thought I'd use on the layout and exercise a reasonable amount of restraint against buying too much excess stuff.
Sure I bought stuff I wound up not using, but there wasn't very much and anything that has value, I can probably sell once the scenery is done and I know for sure what I need to keep or get rid of.
But in the end, I'd say the locomotives were the greatest single cost if you count them all together (even though I bought them over a 4-year period):