How much airfare a mile buys is different than ‘how much a mile is worth’. You can’t spend miles on nearly as many things as cash (and get lower value for things other than travel when you redeem that way). You don’t earn interest on your miles. There’s more devaluation risk with miles.
Here’s a stab at quantifying the discounts that you should consider taking when converting ‘how much airfare do miles buy’ into ‘how much are miles worth’?