Rashford should be making the same money as Ashley Young (which is too much for Young). It shouldn't be anything more. Having said that, it's weird to want Rashford gone. He would be a great squad player if he gets a little better, which is more than possible given that he's only 21.
Our wage structure is awful due to players like Ashley Young, who has had awful performances year after year, which should lower his wage demands should he have left United. This is why players like Martial got the wages he got, and why Rashford wants his wages raised to that level.
Get rid of the deadwood. Send a message that we won't accept mediocrity. Sadly, we would have to pay out Young as nobody will want him on his current wages.