Add price-trends: monthly median $/TB over time (consumer vs enterprise)

Computed monthly medians (Jul 2024 onward) plus a chart. Aggregate only - no
underlying per-drive price observations. Method and interactive version noted
in price-trends/README.md.

price-trends/README.md

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+# NAS drive price trends
+
+Monthly median price-per-terabyte for current 8TB+ NAS hard drives (US, new, in stock), from Jul 2024 to the latest month, split into the two tiers people choose between: consumer NAS (WD Red / IronWolf class) and enterprise (Exos / MG / Ultrastar).
+
+![Median $/TB, consumer NAS vs enterprise](nas-price-trend.png)
+
+## The trend
+
+Median $/TB sat flat around $20/TB through 2024 and most of 2025, then roughly doubled to the mid-$40s/TB from late 2025, and has held there since. Both tiers moved together: among new drives, enterprise is not the $/TB bargain it's assumed to be, and both got hit about equally. The usual driver is the AI datacenter buildout pulling high-capacity nearline supply.
+
+## Files
+
+- [`monthly-median-per-tb.csv`](monthly-median-per-tb.csv) - month, consumer median $/TB, enterprise median $/TB. Computed monthly medians only.
+
+## Method
+
+For each month, take each drive's average $/TB across its logged days, then the median of those per-drive values across drives (so a model with more logged days doesn't skew the month). Scope: 8TB and up, US, new in-stock prices, compiled from Amazon price history. CMR/SMR and specs from datasheets.
+
+8TB and up because small drives carry a much higher $/TB (fixed per-drive costs spread over less capacity), and which small SKUs happen to be in stock would swing an all-capacity median for reasons unrelated to prices actually moving.
+
+This folder publishes the computed monthly medians only, not the underlying per-drive price observations.
+
+## Interactive version
+
+The live, interactive version (hover, zoom, a capacity-matched cheapest-$/TB table, and the full write-up) is at [nasdisks.com/articles/nas-drive-price-crunch](https://www.nasdisks.com/articles/nas-drive-price-crunch/).

price-trends/monthly-median-per-tb.csv

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+month,consumer_usd_per_tb,enterprise_usd_per_tb
+2024-07,19.94,20.93
+2024-08,20.03,22.38
+2024-09,20.38,21.43
+2024-10,20.52,21.84
+2024-11,20.07,21.11
+2024-12,20.74,20.81
+2025-01,21.00,21.22
+2025-02,21.17,21.58
+2025-03,21.00,21.75
+2025-04,20.82,23.24
+2025-05,21.30,21.45
+2025-06,20.42,21.87
+2025-07,20.87,21.48
+2025-08,21.01,20.51
+2025-09,21.50,20.47
+2025-10,22.34,21.17
+2025-11,20.78,22.25
+2025-12,23.67,23.07
+2026-01,27.95,25.02
+2026-02,26.05,27.94
+2026-03,33.43,31.48
+2026-04,39.05,37.99
+2026-05,39.12,39.31
+2026-06,43.52,41.69
+2026-07,44.95,45.31
+2026-08,44.01,42.54
+2026-09,44.37,43.45