If your application is just an information publication application, look at the Notes to Web publishing programs and the Domino support built into Notes 4.5. This approach will also allow your Notes information to be published to all Internet/Intranet users. More information on these programs can be found here.
If you just want to use the Notes applications which are bundled with Notes Express (now known as Notes Mail), using Notes Express from Lotus will let you deploy Notes for approximately $50 per seat in quantities. Any custom applications that use the Lotus API directly can also be run from Notes Express.
If you wish to develop custom applications, but not allow most users to develop applications, Notes Desktop is a run-time version which available for $79 per seat in quantities.
Finally, if your users will always be using web browsers and POP3 clients, user licenses are only $30.
The latest pricing information is here.