Fixed the cost. Hm... I'm going to toy around with a few more ideas besides the nova. Personally, I think that simply increasing the damage would be a bad way to upgrade it, so I'm going to consider any other additions besides the stat change. While a slight stat boost is nice, I don't really see it as being the best of upgraded versions.
The current idea I'm toying with is to give it a "Burning" effect, where it constantly inflicts damage while it's on the field as well as when it detonates. However, the health for the self-burn would need to be taken into account to balance it. I'll keep thinking of other possibilities and see what could come up, though.
Edit: Okay, new idea hit. I was thinking of making it auto-burn itself, but the balance on it would be tricky, and I didn't want to make it a "Destroy this ASAP to avoid automatic burn". So, I put a cost on it instead. I'm currently toying with the idea of what would happen if the effect were usable more than once per round, among other things. I'm currently thinking about what a "Life restore based on damage dealt" alternative might be like, but at the moment, I'm thinking that's a bit much.
Also, sorry about the template swap in the card, the full text wouldn't fit in the old one.