Look at resell history. U will never get back that extra $100 u spent for the 64gb a 16gb iPad Air 2 will still sell for almost the same as ur $350 iPad Air 2 64gb.
The market dictates the resell value. You obviously couldn't sell it for $400. I couldn't sell my iPad Air 2 64gb (wifi also have a wifi plus cellula iPad Air 2 64gb either). But I know full well I can't sell the wifi version for $400. Maybe the cellular version. But I don't bother reselling cause I really don't use my iPad Air 2 anyways these days. My kids do.
Sure u may find that one sucker. But those days are getting harder and harder. iPad resell values aren't what they used to be.
If u want objective evidence what iPad Air 2 64gb resell values are. Go to eBay and look at "sold items". They go from $300-350. Plus around 10% eBay/PayPal fees
Thus $350 is the price point these days