Sadly, there is not a Numbeo app. Based on the design of the browser site, though, this does not come as a surprise to me. I do not get the impression that Numbeo is particularly well funded or rolling in the dough. That being said, the site does work on mobile devices. Once again, it doesn’t provide the best mobile design you’ve ever seen (you may have to do a lot of screen pinching to read things), but it is accessible at the very least.