What has the best cost per use in your skincare routine?

I've been thinking about skincare differently lately. Instead of looking at the price tag I'm looking at cost per use and how long things actually last. Some of my cheapest products run out in a month so they're actually expensive over time. Some of my pricier purchases have lasted forever and paid for themselves. What's your best cost per use skincare item? Could be like anything maybe a product that lasts ages, a tool you use daily, a one time purchase that replaced monthly spending. I want to be smarter about where my money goes because this constant repurchasing cycle is draining me