I find it unworth it upgrading skill attributes that increases their damage or rate of doing something else.
Having to spend some coins and wait some minutes just for 1% of boost is a lot bad.
Upgrading an Attribute should give better bonuses.
Now, one example of attribute that is definitivelly not worth training, are the skills of adding a chance of spawning a tougher or faster zombie when monsters are killed inside the skill area, the attribute gives 1% of chance of spawning a tougher zombie, but at the cost of 68k Silver Coins!?!
Please, revamp the Attribute system, it is definitivelly not worth training for skill damage.
