I make my own soap because dd and I have skin issues. I make a very, very simple lye soap: lye, lard, water. Dd likes to have a natural essence in the soap, so I will put in lavender or rose sometimes.
1 (16oz.) can of lye is $3.89
1 (4lb.) tub of lard is $4.89
I use about 1/2 of a can of lye to a tub of lard. So I'm spending about $6.50 and it makes about 30 bars. I have no special equipment. I use an icing bucket from the bakery, an old wooden spoon and a plastic drawer I pull out for the mold when I'm making soap. This much soap will last us most of a year.
So, for us it's very cost-effective, but more than that it is healthier for our skin.
The essence is $2.49, so at most we pay $9/year for soap.
ETA: my father, who is truly my traditional homemaking skills muse, brought us an entire case of lye (because he knows how hard it is to find) earlier this year. We have enough lye for the next 25 years!
. I keep about 20lbs. of lard around in my pantry (not for eating!!) and the last time I bought it, it was cheaper than I listed above, so in reality, for us, we spend only about $5/year on soap because the lye was free.
Lye is caustic, so it must absolutely be kept dry. Be careful about that.