Probably because Adrafinil is still unscheduled and essentially the same thing, nobody really is that worried about paying a premium for modafinil. I've been buying adrafinil for a little over 2 dollars per dose. Modafinil is better for everyday use (more liver stress with adrafinil) especially if you can get a scrip, but "study aids" are normally not taken every day anyway.
I think you mixed up Adrafinil and Adderall.
Modafinil is now being covered by at least one insurance company in Massachusetts under which it costs less than $1 for a 200 MG pill. I predict a huge college black-market trade in the drug.