This came in the form of a prefab log cabin - that is to say, the lumber is all cut to shape and size, and you're "just" banging it together. "You" in this case being my wife and I.
Total for the structure was just under €20k - includes windows, doors, insulated walls, floor, ceiling.
Total for water is less than €1000 - IBCs for storage, a km of water line, carbon and sediment filters, UV-C reaction chamber, reverse osmosis.
Total for waste is about €200. IBC. Worms. Cheap plumbing supplies.
Total for power is about €15,000 - most of that cost is in a huge bank of OPzS batteries, and then theres victron gear, a dozen solar panels, a dinky wind turbine, and an experimental (failed) waterwheel. This covers both the mill and the cabin, which are about 600m apart.
So, all in all, it's an off-grid house for under €40k. That of course doesn't include labour, which has been about 1000 hours of our time, from digging and pouring column foundations through to transporting everything to site by aerial cableway - so in many cases it'd be much less, as you wouldn't be building on a 45 degree slope with zero access for machinery.
The thing I keep seeing people getting wrong is energy. Literally yesterday I talked someone out of buying used lithium ion cells from EVs, as while they were indeed very cheap, they likely only had a few years of life in them, so the price/kwH stored amortised over the lifetime of the batteries was terrible. Can't go wrong with lead acid OPzS - 20 year lifetime, easy to maintain - two years in and they've lost basically nothing in terms of their potential. For some reason you don't often see them in domestic installs - but they're super common for battery backups at hospitals, battery storage for renewables providers, etc.