The answer is simple: the price is much lower than its cost.
Currently in market 256MB DDR2 cost around 3~4USD, but RPi Zero have 512M DDR2(price should be over 5USD), Broadcom do not make DDR2, they have to buy it from somewhere else, even they can make it to 2.5USD, the other costs still over 2.5USD.
The total cost is impossible to lower than 5USD.
It is a good advertising, but we have to forget it, it won’t product anymore.
PS: … Hope I am wrong. :p