One word of caution about “hard-to-know” data: they are never absolutely correct.
If anyone claims to know the actual price of, say, steel, or chlorine, or paper, or any other “hard-to-know” commodity, don’t believe them. Every deal is different (quantity, quality, freight, etc.). In addition, true “transaction prices” are often deliberately obscured by volume rebates, secret discounts, scrap allowances, etc., many of which do not show up on invoices. As a result, a high degree of pricing accuracy is simply not possible.
However, we always get the direction right. Propurchaser data is highly accurate when it comes to the direction of the pricing (up or down), as well as the magnitude of change (e.g. down 2% or 10 %).
Fortunately, direction is what counts when negotiating pricing with suppliers. Simply knowing a supplier’s costs have fallen is usually enough to re-open negotiations.