The amount you would get for the scrap cast iron bathtub at the scrap yard would depend on the price per ton that the scrap yard is paying for scrap cast iron.
Here at the scrap yard in my area they're paying $110.00 per ton so you might get $10.00 to $20.00 out of the scrap cast iron bathtub.
It's not gonna make much but will likely pay for your gas at least but if you don't want to do deal with it you can always put it out by the curb and someone who scraps will likely haul it away for you for free.