Yer its already implemented on the economy page.
But do you mean the profit from when you bought the player?
So if you bought for $100k, and are selling for $80k, then you want it to show '-$20k loss;'
which, if my understanding is correct, I think would be a nice idea :)
No, that was not what I meant, but I suggest you'll open another thread for that, as it seems a good one (but even with a lower priority than this one).
Here I've meant that when you are setting a player's auction price (when you want to sell it), you know that for example you asked for 100K (for example) and will get 97% of the revenue requested (for example).
So, I suggest writing it down: "Requested price -100K. Estimated value upon this price - 97K"
Of course updating the current bid's value that a user will get for a player is to be added as well.