I'm getting the same deal when using that formula - everything is "-". What if you do something like this instead and show a trend icon?

Previous Mo is the Previous Month metric, which is =Previous Month(Cost)
Current Mo is Cost, and then I right-clicked on the header to rename it
I have a Variance Step1 column that I later hid (right-click the column name in the Values area and select Hide)
Variance Step1=Abs(Cost)-Abs(Previous Month Costs)
Variance True (later renamed column to just Variance) is:
Variance True =If(Previous Month Costs=0 AND Cost<0,Cost,{Variance Step1}
Trend is another calculated column that was created on the report itself:
Trend ="<div align='center'>"&Icon("3arrows",Variance True>0,Variance True=0,Variance True<0)&"</div>"
@Matthew Sparks
#ApptioforAll