I'm not sure about potatoes costing less than onions, but can see why they could add bulk (potato also used as a thickener in other types of cooking, e.g. soups). I'm wondering about what the minimum ingredients would be for a decent base, with the flavours being added to individual curries at the cooking / assembling stage. I like the idea of maybe experimenting with a base that has only onions, garlic, ginger and water.
What do you think?