AFAIK it's never really discounted in relation to what it would be if you had booked one way in Business from the outset, and of course it's totally subject to availability at that moment. By all accounts the Premium cabins seem to be pretty full on a lot of flights right now (also my own experience recently on KLM and Delta between Europe and USA).

If you look at the miles price for a one-way in Business on your travel dates that may give you an indication. Of course others will have recent data points too.