I don't remember where I read it, but I remember reading a magazine article that said if you wanted to keep track of how old your makeup/skincare is, you should mark the container with the date you started using it. That way, it's easier to know if you have really and truly had something for too long.
I thought that was a great idea, so I started doing that. But then - because I get obsessed with some things quite easily - I decided to take it a step further. I mark things with the date I start using it, and then when it's used up, or I'm ready to throw it away for whatever reason, I calculate the approximate cost per use. Then I can compare it to similar products to determine - all other things being equal - which product is actually less expensive in the long run.
I have explained this many times to friends, some who find it just weird, and others who have started doing the same thing or a version of it themselves. My friend Lisa christened this the BMST, or Bridget's Mathematical Spending Theorem.
Let me give you an example of an actual comparison between two products. A few years back, I treated myself to a [somewhat] high-end foundation that was supposed to be good for people who have rosacea (which I do). It was $40.00 for a 1-ounce bottle. I generally use foundation every day that I go to work, so for the sake of argument, say that I used it approximately 250 days over the course of the year. That turned out to be $.16 per use. Pretty good, right?
Well, then I bought a drugstore foundation, which was $12.00 for a 1.7-ounce bottle. Certainly that is a better deal, right? Well, it was gone after 5 months (approximately 100 work days)! Turns out that I had to use more of it to get the same coverage I had with the other stuff. (Basically I like my face to look evened out once I put on foundation, to give you an idea of how much I do/do not use.) That turned out to be $.12 per use - which appears to be a better price, but - I had to buy another bottle to finish out a year, so that's nearly$.24 per use, which means that it was actually more expensive.
I have to tell you, I was really surprised to learn this! It proved to me that cheap is not always cheaper all around. And expensive can be more economical.
Granted, there are some things where the result is even. There are also some things that I use that even if they are slightly more/less economical, I stick with them for other reasons, i.e., they work the best for me.
If you are trying to figure out where your money is well-spent, though, you may want to give the BMST a try. I'm willing to bet that you are in for a surprise, or at a minimum, a better awareness of how you are spending your hard-earned money.
**The first year I belonged to a gym, I would mark on the calendar each time I went for a workout. Then at the end of the year, I was able to determine how much I was paying for each visit, and decide if I wanted to continue. (Yes, I need a life ...)