I just had some nice success doing a cinnamon extraction!
I used a 1tsp to 10ml pg ratio. This seemed to work pretty well for me. I started with a 3tsp/30ml mixture, and ended up with about 20ml of extract after it ran
through a coffee filter. I ran the extract
through again, and ended w/ approx. 18ml.
Clove is another spice that I think might work very well via this method.