Google trends now tracks sales of any product being sold by Google Checkout, and Google Apps Premier Edition is sold exclusively using Google Checkout:

The chart shows a step drop in sales for Google Apps Premier Edition starting in mid December and not picking up in the new year. As noted by Google Blogscoped (who also spotted the chart), other products show a decline around this time meaning that it could simply be a case of flawed data in the trends tool itself, or simply its a seasonal thing. Either way you can track sales by going directly to the chart here. If it doesn’t pick up again by the end of the month we’ll start to ask some deeper questions as to the success or otherwise of the paid version of Google Apps.