Is there any way to estimate the weight difference between a trimmable horizontal stabilizer and a fixed tailplane? I am trying to estimate the benefits of using a trimmable horizontal stabilizer (THS) instead of a fixed tailplane. In my model I use a T-tail (Sh~30 m), and I can trim the aicraft in every condition so the main reason to go for a THS would be aerodynamic efficiency.

I've estimated the drag reduction but I'm having huge problems estimating the weight increment of the system. 

Please, could someone shed some light on this? I've checked a lot of references but still couldn't find anything clear.
Hard to find a general reference for this one. You're adding the weight of the hinge, the actuator, and the supporting structure now connecting only to the hinge axis.

Torenbeek section 8.4.1 gives a weight estimation for preliminary design purposes for the tail group of high subsonic aircraft: it mentions a weight increase of 10 % over the weight of a fixed horizontal tailplane.