You should not support cryptocurrency miners by buying their old cards.
Otherwise, the usual way in accounting to calculate the value of an asset is to divide the purchase-price by the number of years of expected lifetime and subtract that every year it is used.
So, if the expected lifetime for a new card is 10 years, and the used card is expected to last one year, then it would be worth 1/10 of the original price.
The GeForce GTX1070 had a MSRP of $379. A fair price for that type of card used in cryptocurrency miner rig would therefore be $37.90.