Single item? Probably the Maxx Air deluxe roof fan. It was about $250.
If you want to talk about purchasing things in logical batch sizes or categories to complete a particular part of the project or make a complete purchase, that's another thing. When it came time to order the big ticket items, in no particular order: 30 yds of 6oz fiberglass cloth was about $9/yd, but I have bought a yard or two more here and there, so about $290 in glass cloth. The total bill for all epoxy and composite related products is probably around $700 at the moment, including resins, hardeners and fillers, a good lot of that was plunked down at once, about $450 worth (so comparable to aluminum skin and trim).
If you add up the axle, brake hub assemblies and related hardware kit that was a $450+ nut to crack. The Rough Stuff/Jeep suspension (springs, shocks, spring hanger hardware, etc) was close to the same by the time I added it up.
$600 for 4 sticks of 2x10 x 10 ft select cedar, straight grain, zero knots that got ripped down and became my inner floor and wall frame work. I never once regretted the quality of that purchase... it has been a dream to work with it. Another chunk close to that went to the 5mm Okoume marine ply shipment (IIRC, 13 sheets worth). Again, no regrets.
The ATX Mojave wheels and 31 inch Toyo tires were about $150 each, so another $600 nut, but I guess all of those count as multiple pieces.
The portable folding solar panel pair seems like it was a bargain by comparison; I think that was $130 for 130w (little fuzzy on the price at the moment, maybe it was $200? ...I have the receipts).
Haven't bought the mattress yet, or final paint supplies. Those should be the last big ticket items left. (

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Did I mention that I am way, way over budget, about 40/ct over my initial top end estimate? But then again, it has taken almost 4 yrs to get here, so it doesn't hurt as much as one might think it should.
Like Big Mike says, use the best materials... and like a well respected TD manufacturer once said, (paraphrasing) before you complain about the asking price of a custom manufactured camper, consider all of the things that go into one to make it the reliable dream camper you expect.
Admittedly, I have splurged on almost everything that has gone into TPCE. Probably not a wise financial decision, but so much more of the value is in the labor of love. That only costs me time.
Slow, I think it is all relative. My advice to any builder would be to plan your build to fit your budget. Save for the things you "must have" and take your time building so that the bills don't all come at once. If it takes another year to save up more funds, spend the extra time in planning and labor to make each and everything that little bit nicer, that extra little bit special. Like including shocks.

Also, unless you are a very efficient builder (like a cabinet shop owner or something similar) maybe it would be better not to get so hooked on this hobby as an emotional calling. Putting more money and labor into your passion than it will ever be worth on the open market is not a wise financial decision. It will never pay itself back in hard funds, only in the inner happiness that it brings to you. Choose wisely.
