Unfortunately the PlayBook, unlike other BlackBerry devices, doesn't directly support applications that aren't released through the AppWorld.
There are a couple of ways around that.
You could put every device in developer mode and then connect via cable directly to the development machine and deploy the application. For a very small number of devices this is probably acceptably simple. For a larger number this isn't very practical.
You can sideload apps onto the playbook by connecting the device via USB cable and using a few simple tools. This blog post explains it: http://limetrip.blogspot.com/2011/09/how-to-install-angrybirdsbar-apps-to.html
The above method works but you still need to physically connect every device to a PC. There's an application that's been developed called LocalBar that allows you to sideload BAR file directly from the PlayBook. However, the LocalBar app itself requires sideloading first. There are instructions here as well as a download link: http://limetrip.blogspot.com/2011/11/download-local-bar-app-for-free-install.html
Costs: in either case (development mode or sideloading) none, except the manual labour involved for each of the devices.
License issues: developer mode is completely fair game and doesn't break any licenses. It's a pain though because you'll need to create a developer certificate for every device. That's not hard, but multiply any of this by 20 and it's a hassle. I'm not sure about the license issues related to sideloading. It should be fine, as long as it's your own app.
Limitations: none (as far as I know)