I wanted to break it down for understanding.
You can find Nutrition facts from one of gluten free cereal from a leading brand below: (source: https://www.postconsumerbrands.com/wp-content/uploads/2016/12/Pebbles_CinnamonPebbles.pdf)
This cereal contents two sources of carbs: a) rice b) sugar. As per nutrition facts, every 30 gram of cereal has 10 gram sugar. Essentially when you buy 15 oz of cereal (425 grams), it has 5 oz (140 grams) of sugar.
But that is deceptive.
Walmart sells Great Value cane sugar at price of $3.02 for 4 lb, which is 64 oz. Which means price of 1 oz of sugar is $0.047.. 5 oz of sugar comes to $0.23 (mind you, i have not pickup corn syrup or non-cane sugar which may be even cheaper - post doesn't indicate which type of sugar they have used)
At $3.12, you are purchasing 10 oz of serial and and $0.23 worth of sugar.
Your net cost of cereal is $2.89 for 10 oz of cereal (and $0.23 for 5 oz of sugar). Essentially your cereals are costing $0.29 per oz (consisting of mostly rice).
Almost all cereal manufacturers are to blame for such products .While there is potentially no mechanism to make a gluten free cereal taste good - pumping it with sugar should not be the way to do it.