It really depends on the design you want to purchase. Mechanical choices can cost a pretty, which is to say that they could even get as expensive as one hundred dollars. There are reasonably priced options, though, most of which are priced at around forty to fifty dollars.
Electronic metronomes, by comparison, cost a lot less. One of the most popular models we have stumbled upon cost just thirty dollars at the time this article was written. Better yet, it was also a tuner, not just a metronome, so a winner in terms of versatility.
As for apps, they can cost anything from two dollars to whatever you feel comfortable spending. They are by far the cheapest alternative.