How often you need to use it to make it worthwhile: Once a week for two-and-a-half months.
Yes, you can make yogurt with a pot, a thermometer and a jar—and people have been doing so for centuries. But a machine, with its built-in timer and automatic shut-off, can make the process insanely easy (also, we are total suckers for the little jars that come with it). If you buy a 6-ounce, 89-cent container of strawberry yogurt for breakfast or a snack every day, in 80 days you'll have spent about $71. Make your own 80-day supply, and you'll have laid out $40 for the machine, plus $33 in ingredients—so $73 total. (Caveat: If you have a slow cooker, you can make yogurt—so there's no need to add another appliance to your counter.)