We must leave the obsession with the iPad aside for a moment. The whole idea is to deliver educational content through a hand held device. The thing here is there are many parameters attached to it. First is the device, then the content delivery itself has a cost(Internet subscription et al) then the maintenance of devices etc. So the cost of the device is not the only thing. There are many incremental costs.
Most of this is definitely out of the reach of the people who actually need it. $500 is a lot of money for people in my country. Most may be earning only 1/5th of that in the whole month. Average salary of a teacher in India a around $150 a month. Which is around $5 a day average.
How do you expect mass technology adaption with these sort of expenses?