Technically, $250 upfront if you're look at street pricing for base models (so more or less enough savings to buy a new entry level model sooner if needed). Besides, chances are if you resell, depreciation on a $250 iPad 6 will be less than depreciation on a $500 iPad Air 3.
There's also absolutely no guarantee one will keep the Air 3 longer than one would the iPad 6. There are reasons other than pure horsepower that one might wish to upgrade. Newer models might support 8K video, a new codec, new connectors, faster wifi standard, 5G, etc.
Note, it's possible for the entry level iPad refresh to get A12/3GB as long as overall specs are still lower than Air 3 (so likely lower storage, no Smart Connector, no laminated display, no anti-reflective coating, no True Tone, no eSIM, worse cameras, etc). We did get the A9/2GB iPhone SE which had same CPU and RAM as then latest iPhone 6s for $200 less.
As for the produce argument, we've actually stopped buying bulk packages of goods that can spoil. If you buy the $3.99 bag at $0.25/lb and only eat 4 lbs (or less) and throw away the rest, you've still spent $1+ more than you needed to.
This is also akin to telling someone they should spend $100-150 extra for 128-256GB storage when that person wouldn't even use 5GB.