Calculating gain of Amplifiers
I see you are talking of the resistor from source to ground, and the total resistance on output for the gain of a common source/emitter 1 transistor amplifier. 

As regards the sign, I don't think the equation takes it into account as the common source/emitter amplifier is an inverting amplifier. It is just the absolute gain. 

Of course you could specify the equation as $$ -\begin{equation}  \  Rdrain \over\ Rsource \end{equation} $$ to make sure you always get the right polarity. 

In fact (just noticed) if you check the Wiki page on Common source the (more exact) equation is as follows: