The Nikon is listed at ~80% of the price new, Tamron is same new and used. You can't compare to retail price. When lenses get replaced and/or phased out and the manufacturer changes the price, everything changes. "Value" has to do with Current New vs. Current Used, not what you paid for it (though it would be great if things never went on sale 3 months after you buy them). There's been a recent history of new products having their market price drop rather quickly after introduction, so it's the cost of being first on the block.
BTW, except in rare cases, 3rd party lenses like Tamron & Sigma don't hold their value as well as Nikon glass, even for the consumer grade lenses.