I use my thermometer only to be sure my soap is below the flashpoint of my EO. I hate wasting that stuff. And I had a whole batch become unscented(orange EO) by not testing it.
My wasted money:
Some EOs. Lots of wasted money in those. But, I could not smell them in the sealed bottles, so at least I learned something.
Exotic oils. I now know I can make perfectly good soap from oils I get from Walmart. I am scrambling to use some of these up before they go rancid.
My best tools:
$1 spatulas and spoons from Dollar Tree. I adore that long handled spoon and that double ended spatula.
$1 blue drawer divider "molds", again from Dollar Tree. I got 2 of them, and going back to get more soon. Absolutely perfect for 1-3 lb batches.
My stainless steel bain marie. I mix every batch of lye water and then soap in that. Tall and skinny, it is just perfect for the stick blender.
My stick blender, crock pot, and scale are absolute necessities.