Boxed (or packet) cake mixes are convenient - to a point. You still need to add the wet ingredients, usually egg, water or milk, butter or oil, so what you're paying up to $9 for is basically flour, sugar, a rising agent, flavouring of some kind and then a whole lot of things you can't pronounce and really shouldn't be eating.
That cake mix convenience ends up costing you up to $11!
If you make a cake from scratch you'll use flour, sugar, butter or oil, milk or water, an egg or two, flavouring of some kind and if you didn't use self-raising flour, a rising agent (baking powder or bicarb soda, depending on the recipe). And your cake will cost you under $2!
Now cake mixes are supposed to be convenient time savers because you just dump the contents into the mixer, add the wet stuff and beat for 3 minutes.
If your recipe is a one-bowl mix, like the I've shared below, you do the same thing - dump all the ingredients into the mixer and beat for 3 minutes.
It may take you a minute to measure out the flour, sugar and flavourings so a from scratch cake will take you 1 or 2 minutes longer to get to the baking stage. At a saving of up to $9 a minute or two is nothing.
And they will both take about the same time to cook too, so no saving there.
If you really love cake mixes, and I confess I do, you can quite easily make your own.
When I'm baking a cake I get out a half a dozen ziplock bags and measure out the dry ingredients for 7 cakes, the one I'm baking and 6 to put in the cupboard. The bags are labelled with the type of cake the ingredients will make - chocolate, coffee, butter, sultana, cherry or whatever, a list of wet ingredients and the instructions. I do this a lot so I have marked the bags with a Sharpie. Once the mix has been used the bag is washed and dried and put away ready to use again - and the instructions are already written on it.
This is a quick chocolate cake I make into MOO Cake Mixes.
Quick Chocolate Cake
3 cups SR flour
2 cups sugar
1/4 cup cocoa
2 cups cold water
2 tsp vanilla extract
2 tbsp white vinegar
3/4 cup vegetable oil
You can double, triple, quadruple the quantities (I measure out 7 lots at a time). Put the dry ingredients into ziplock bags and seal. Label the bag and add a list of the wet ingredients and the instructions.
To make a cake add the wet ingredients to the dry ingredients. Stir until just combined.
One quantity will make 2 dozen cupcakes or two 20cm square cakes. Bake at 180 degrees Celsius for 25 - 30 minutes for 20cm cake (or until a skewer inserted in the middle comes out clean). Bake for 12 - 15 minutes for cupcakes or until a skewer inserted in the middle comes out clean.