For B-vans, they're generally representative if the van is five years old or newer. NADA is based on reported selling prices. Of course, dealers use NADA for pricing their vehicles, so it's sort of a self-fulfilling prophesy in a circular-logic sort of way.
Older than five years, and most of those sales aren't reported to NADA at all. Private sales, of course, aren't reported... so they must use some kind of depreciation algorithm to come up with the prices.
Prices of vehicles ten years or older are based on condition, comps on the market, and what the market price will support. I've seen vans like mine (twenty years old) with a similar age vary as much as $20,000 in asking price from $5k on the low end to $25k on the top. Of course you can get $20k discounts off sticker on $150k MSRP vans... so I guess it's all a matter of perspective.
FWIW, for the first time this week I saw an ad with the sticker price for a B-van top $200k.