For what it's worth, max level for 'element buildings' is level 10. There's a bit of a jump between 9 and 10 too.

Level 9:
- +125% element damage
- 125k gold to upgrade
- 7 days build time to upgrade

Level 10:
- +200% element damage
- 250k gold to upgrade
- 14 days build time to upgrade

I'm a F2P player and bought the multiple upgrade thing from the gem store for 1200, running the Hard boss zones 1/day and doing Quests for gems (still over 1300 left) will generally be enough to pay for the upgrades. I've got three different ones upgrading to 10 right now, one's still on 9 and the last one finished upgrading already. Once that's done all that's left is item storage and such so i guess that's the end of silver as a useful resource aside from Skill upgrades. Hard to beat a +75% damage boost though, and it should affect their attack skills as well.

Something to keep in mind if anyone else is considering buying the Multiple Upgrades thing. I can't be arsed to wait two weeks for a building to upgrade so i picked it up, but YMMV.