What is the cheapest starch? Use dry weight, or bucks per calorie.
This tends to maximize for the calories per acre of land used, and the costs to bring it to market.
It does not take into account externalized costs of farming: Soil loss, chemical pollution, habitat destruction.
So your next step would be to start with a list of cheap sources,then to look at how each is typically farmed.
So, for example, potatoes are cheap, but the chemical use for spuds is unreal.
Soybeans are high chemical users too.
Grains: Wheat, barley, oats are only moderate chemical users. I would give serious consideration to any of these three that you could buy organically, especially if you could buy whole grain at the farm gate. Look also for organic peas and beans.
As a starch source, oats is the highest yield per acre. Hulled oats are easily chopped into scottish 'steel cut oats' which have a fairly low glycemic index, so they stave off hunger longer.
This pattern could result in dietary issues. Most grains have protein but it is incomplete. If you are using this a supplement, fine, but if it's going to be a major part of your diet, look at mixing sources to get the right protein mix. Beans and either rice or corn work well. Possibly any legume combined with any grain. The vegan circle has good information about balanced combinations.