A quick search revealed that quinoa is between £4.50 and £8 a kilo in the UK and brown rice is £1-£4 (depending on quality).
To the rice you could just add some bran (£2/kilo, but you don't need much) and another, cheap source of protein to make it similar to quinoa's nutirtional values.
Between 2006 and early 2013 quinoa crop prices have tripled.
That statement cites an article earlier this year in the Guardian [1] which links to another Guardian article [2] which doesn't have anything more specific than $4500/ton in 2013 versus $3115/ton in 2011.
To the rice you could just add some bran (£2/kilo, but you don't need much) and another, cheap source of protein to make it similar to quinoa's nutirtional values.