The cost of the basic layer cake, per layer, is tied to the type, quality and quantity of ingredients you put into it. Based on where I do my shopping (Fresh & Easy in Long Beach), one layer of the basic layer cake costs me $1.28:
In comparison, the cheapest box of yellow cake mix I found at the store was $1.79, which would require the addition of three large eggs ($0.50, based on $1.99/dz large eggs) and 1/3c oil ($0.22, based upon a cost of $3.99 for 64oz). If I went to the dollar store, I could find the same (or an imitation thereof) for (drumroll, please) $1.00, with the same required inputs. Since I can make two 9-inch layers with that box, my cost would be between $0.86 and $1.26 per layer.
$0.86 per layer for a box of chemicals and preservatives that I have to go out of my way to buy; pay for a unitasker ingredient; have no control over flavoring; and no means of small-batch scaling…versus $1.28 per layer for homemade. Shit, I will happily pay the extra $0.42 per layer and call it a day.