Amy Dacyzyn, who wrote The Tightwad Gazette, gives some good guidance here. She talks about what your total return per hour is on doing something frugally. If changing your own oil takes 15 minutes and saves you $15, then you "earn" $60 / hour doing it over time. Notably, she doesn't really recommend couponing as an activity, but does recommend shopping sales and buying in bulk, using an extra freezer to keep low-price meats and then "shopping" in your own freezer for meal planning.

A lot of rich people became rich by being frugal (along with having a higher income). But there's such as a thing as false economy. It's up to you to determine true vs. false economy in your life.