I believe you, and this is clearly a bug, but I can't seem to reproduce it here: for me, entering a value of "12" gives me a result of "$12.00". (And just to be certain, I tried your examples of 42.50 and 780 and got back "$42.50" and "$780.00".)
It might have something to do with your Currency format settings in System Preferences (under International / Formats): mine is currently set to "US Dollar". Or perhaps it's related to something else in your environment.
My advice would be to send feedback to Omni's technical support ninjas by selecting Send Feedback from the Help menu (or just sending email to
omnioutliner@omnigroup.com): hopefully they'll be able to help you diagnose and work around the problem. (And if they can figure out how to reproduce the bug here, it will make it much easier to track down and fix for a future release.)
Thanks!