I wonder if you're thinking of a magic site that provided a discount to casting the spell? I think that would work. And if you only plan on it staying up one turn anyway that could work out nicely. But if you plan on keeping it up then you'll undoubtedly want to put extra gems into the casting and the discount site won't matter since you won't get credit for the extra gems until you get past the base price of 80 gems.
Anyway, the casting cost is no big deal either way. One turn worth of discounted forging alone will make up for your base cost.
Re: CBM changes...
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