The official rules state that "in order to lift the mortgage, the owner must pay the Bank the
amount of the mortgage plus 10% interest." The loan amount is the mortgage value, so it seems reasonable to interpret this as 10% of half the price, or %55 of the list price in total to unmortgage.
Right now it is implemented at 10% of the list price or %60 in total.
https://github.com/intrepidcoder/monopoly/blob/15e8a09ab78044e67007a07b5f7dbae88578a8a7/monopoly.js#L1588
The official rules state that "in order to lift the mortgage, the owner must pay the Bank the amount of the mortgage plus 10% interest." The loan amount is the mortgage value, so it seems reasonable to interpret this as 10% of half the price, or %55 of the list price in total to unmortgage.
Right now it is implemented at 10% of the list price or %60 in total.