In my area (South Jersey) 1/2" homosote is about $ 30 for a 4' x 8' sheet and carpet padding is anywhere from $ .49 to $ 1.09 a square foot. That works out to $16 to $32 for the same size depending on thickness.
So padding could cost half the cost of homosote. Maybe cheaper if you find a sale. The prices were from my local depot.
Sean