In case of 50-50 fault when one car has no damage and other has substantial damage does each pay for his own damages or for half the cost?

depends on the liability statue for that state if a pure comparative negligence state then each party pays their portion of fault to the other, in the example you gave no payment of course if no damage, other party (insurance) would pay 50% of the damaged persons claim ...need to know the statue-no fault? comparative? 51/49? pure comparative? etc