The 15oz (pre-shrinkflation) Betty Crocker mix weighs 432g, 2/3 the weight of your recipe's flour and sugar alone, so your recipe is for a much larger cake.

Wal-Mart's own brand flour is about (Canadian) $C0.15/kg if buying 2.5kg, cake & pastry flour more like 24 cents/kg. Granulated Sugar is C$0.15/kg. I'm not sure where you're buying flour & sugar.

Your most expensive ingredient is butter but you could have easily chosen a recipe without butter. I reckon the cake with butter will be much better than BC's version though.

Other advantages of baking with a recipe are being able to scale the recipe up or down as needed, and always having the ingredients for many different recipes on hand without having to buy a vanilla cake mix, a brownie mix, a pancake mix, a waffle mix etc.

The main advantage of a recipe is tweakability. I find most commercial cakes far too sweet, and when I bake from a recipe I can find the point to which I can reduce the sugar without it becoming too bland.

I don't know whether they're harmful but I'm quite happy to cook a cake without Propylene glycol, Xanthan gum, cellulose gum, sodium stearoyl lactylate, monoglycerides, and monoesters of fatty acids

I've always been baffled by the popularity of pre-made recipe boxes. Maybe useful if I was camping or something, but not when I have access to an actual kitchen.