On the RightNote website http://bauerapps.com/ :

HUGE SAVINGS FOR A LIMITED TIME ONLY - 50% DISCOUNT UNTIL 16TH NOV

Sadly, not applicable for version upgrades...

I use Notecase Pro, which runs on Linux, Mac, and Windows. It's cheaper, full-featured, robust, and under significant development. I haven't used Rightnote. But what's the advantage? I assume it has one. Or several.