I'm unfamiliar with Fate, but wouldn't that mean that there'd be no point in having Excalibur on the CS in the first place if it's too expensive to ever use?
I mean, if it's that draining, then just summoning it would deplete her mana and cause her to cease to exist the second she tried it, right? So, like, that's not even a suicide attack, really. It's just flat-out suicide.
That seems a bit silly is all. You might want to trade it out for something cooler and more usable.