'cost'  would be the price of the diesel engine,  and  any  'parts'  needed to make it a  tractor engine: differences  in   manifolds,  and  water pumps   between  a  combine engine and a tractor engine,  if that's the route you take..  

Early  4020's  with 2 hydraulic valves on the fire wall,  will hit ,, or  be  extremely close to the  exhaust manifold of a  turbo'd   diesel.    1 box  will  clear.