Looking at EBay you can pay around $100 for a secondhand one that has not been cleaned. Asking $50 seems reasonable when compared to some.
The cooling system sends the water through various metal components that react and cause a dirty system. Not everyone flushes and cleans their systems regularly so that is why you end up with the rust stained inside of the coolant recovery side. The windscreen washer side usually contains clean water and often some cleaning additive.
The outside of the bottle can have oil, grease and even road grime attached. There is also the possibility that under bonnet temperatures could cause the old bottle to become brittle over the many years.