Surely people who can afford Soylent can afford to buy decent ingredients, a rice cooker, and a crockpot right?
The average per person spending in the US is somewhere close to $300 per person if I recall correctly. Taking that into account with the time savings, and it's really quite cheap as a pure fuel source.
It matches up with the "Low cost plan" from the USDA for 2014: https://www.cnpp.usda.gov/sites/default/files/usda_food_plan...