Replace exact one null in column.

Anonymous you can't replace it but you can add a new column in PQ or DAX with a logic that will create the column you need. The logic will require that the row will have something that will distinguish it from the other rows in your table

Are the values in the other columns unique? Meaning could you create a conditional column (either in Power Query or data view) to add in your value? Something like this: = IF( [unique column] = ABC), 25000$, [Price]