Yes, for the bargain basement price of 108 skirmish marks, you can upgrade your soldier’s power-over-time spell by 0.8 power every 3 seconds. 108 marks is about half what you get for a skirmish with the daily bonus. The herbalist likes to cast Nature’s Power on itself at the start of a fight, although it may decide to give you a recharge once its cooldown is done.
Looking at the other skirmish trainer options past rank 10, this seems typical. I could also spend 132 marks to upgrade the herbalist’s armor by 6. Six. Skirmish training is hard-capped at 20, but it seems soft-capped at 10. Lower ranks increase quickly and cheaply, which was the stated design intention, but the curve becomes a wall in the latter half.
It is a microcosm of MMOs generally: early on, large and quick increases in power; once you feel committed, increasingly small increments for increasingly large investment. On the other hand, I guess that encourages me to spend my marks on those Empowerment scrolls.