I have returned with the math on Sylvanimora’s ability.
I found that the cost of 17 was acceptable compared to Behemoth’s 14. The following are all approximations but close enough…
Assume that on a given board, there are about 9 of each color (including skulls). Behemoth destroying 10 gems will therefore get one skull and 9 mana. Maybe 2-3 of that 9 mana will start recharging Behemoth. Comparatively, Sylvanimora will explode 24 gems, of which 2-3 will be skulls, leaving 21-22 mana - at the 70% rate for explosions, the gain is about 14-15 mana (4-5 going to Sylvanimora).
Behemoth does +1 damage*, Sylvanimora gets a decent amount more mana even after the extra cost. However, Sylvanimora can also control the columns to destroy - meaning the player can choose which mana to get (potentially recharging half or more of the cost), leave a skull match for the entangled first enemy to try to use, and so on.
So I think 17 is fair.
(*: 16 damage instead of 17, late-game, doesn’t mean much either - easy trade for board control effects)