Depending on how simple the app is, if you are a web developer, you can try Phonegap. It's a good way to test out some ideas and iterate before going full bore on creating custom code for each device base. And Windows phone? I can see if you have a huge Finnish audience, but WP in USA is tiny.
If the developer has even a passing familiarity with how CSS and HTML5 work, then even Phonegap is unnecessary. It's not rocket surgery to build a site that renders well in a mobile viewport. It gets hairy when the site has to be responsive and there are elements that don't scale well involved (ads, charts, videos, PDFs, etc.); but if the target is exclusively or primarily mobile, then you either don't use those elements or you size them for mobile to begin with.
The real question, as masloki points out, is what information the application needs from the phone. If it needs none other than what the browser is willing to provide, then no app is needed. Just build a mobile-friendly Web site. It will work cross-platform, doesn't need vendor approval, and doesn't need to be installed on the device. If a fee is involved, it can be done using a login.
On the other hand, some users (especially younger ones) prefer apps to Web sites. Some mobile "apps" are really little more than shortcuts to Web sites for that reason. But they're still a lot easier to code and maintain than full-fledged, platform-specific apps.
Getting apps approved for each platform and maintaining them costs money. If something can be done as a generic, platform-agnostic Web application without that ongoing expense, it makes sense to at least look at that option.
Rich
