If it is just the digitizer thats cracked (the top glass) then you are looking at a cheap repair but if its the actual display the LCD then the repair gets quite expensive.
Nokia Lumia 1520 Digitizer: http://teksave.co.uk/Nokia-Lumia-1520-Black-Touch-Screen-Digitizer-Glass-Lens.html
Nokia Lumia 1520 LCD & Digitizer (FULL FRONT): http://teksave.co.uk/Nokia-Lumia-1520-Complete-Lcd-Screen-Display-With-Touchpad.html
Nokia Lumia 1520 LCD & Digitizer (FULL FRONT, INCLUDING FRAME): http://teksave.co.uk/Nokia-Lumia-1520-Complete-Lcd-Screen-Display-Touchpad-With-Frame.html