For the past two years, I haven't had to buy soap; I've been able to enjoy making and using a simple three-ingredient soap (one of the ingredients being simple water) - by my calculations, it costs approximately $2.00 per bar, and single one lasts for a solid month. While anecdotal evidence is only the evidence of an anecdote, I have had nearly no skin trouble since starting to use it.
I dislike the notion of putting a lab's worth of chemicals on my skin; we know how well chemicals absorb through the dermis, and paying handsomely for the luxury of doing so has not appealed to me in the slightest.
I recall from stories told by my parents of the various shortages and deficits that were a regularity in their youth. Should the inventory on the shelves evaporate overnight, one'd be able to make this soap with animal fat (preferably avian, but beef is acceptable), wood ash, and water.
For my prior batches, I've used coconut oil, and olive oil - the latter being a time-tested recipe, highly famous in the Nablus region of the West Bank in Palestine - said soap produces almost no lather, but has excellent cleansing qualities. It has been produced there for hundreds of years.
My next batch will be made with sunflower oil, but the principle is all the same. You require some manner of lipid, a hydroxide, and water. When the three are mixed, with the addition of heat, the saponification process converts the ingredients into the soap we all know and love.
Here are a few of the uses