Here is an example of how calculating a cost per use can help. Be warned, though, that cost per use doesn't work for everything. For example, you use life insurance only once, making its cost incalculable.
Subscriptions: Consider a DVD movies-by-mail subscription, such as Netflix. Is it the best option compared with going to the video store, using pay-per-view from your TV package or paying for a movie channel such as HBO? Those aren't exactly an apples-to-apples comparison, but cost per use can help you evaluate them.
With Netflix, you pay a flat fee and keep movies as long as you want, but depending on the package you buy, you might not get another DVD until you return one. The faster you watch them and ship them back, the lower your cost per movie.
In the case of Netflix, a handy online tool is available free at FeedFliks.com. It can link with your Netflix account and report on your use, including cost per movie. That makes it easy to compare.
A fitness club membership is another good example. A membership that's $60 a month costs $2 per use if you go daily. But it's 30 times more if you go once a month.