I have just this quandry going on now.
I use sow/weaner nuts 16% protein from conception to slaughter!! My GOS do well on this.
But Hi-Peak are stopping doing their 'Natural' range which i have used for sometime now, no GM nasties or dodgy by-products. They are going completely organic which would add £100/tonne to my usual order!
I have had great trouble finding a new supplier that does not use any GM ingredients particulary soya. It looks like the Farmgate range by BOCM Paulls might be my answer but we shall see. Farmgate also only do 20kg bags as opposed to 25kg bags i normally get so i think its going to work out more expensive. i will have to do some sums.
Just when you think you've got it right...............
Farmgate is sold @ £7.75/20kg bag so makes it £387.75/tonne,so puts my cost up by £91/tonne 
Might as well buy the organic!!!!!!!