Isn't wheat in the same category as corn for farm subsidies? The long & short of it is that American farmers get government funding to produce these crops - which means that they are NOT being produced at fair market value, they are cheaper to make than others.
Plus there is a whole infrastructure in place that makes it cheaper (from seed procurement, to processing from the field, to travel/distribution), you'd have to start fresh with quinoa. for example, what type of farm implement would you have available for large scale processing?
I'm just drawing a random example, for all I know a farmer can use wheat threshers & combines to do quinoa - but my point still holds, even if on the processing & production end it's the same cost, the Am govnt provides payment for producing the mainstream crops (for example, if it's a low price that year due to bad weather, the farmer will get a check in the mail for the full price - you will not get that check with other crops).
That's why junk food is cheap too.