The initial damages are whatever Zack's contract with Antarctic was, for starters. Then add future work with them. Then potential derivative work that may have come from that relationship. Determining those numbers are how lawyers make big bucks. [EDIT: plus everything AnOminus mentions above]
The direct damages are what's necessary to win the case and actually get anything out of it. Those are very easy to establish.
The other things would be incidental or consequential damages, and he doesn't have to prove those. They're at least partly speculative. Presumably, he'd argue that he'd lost future work, the opportunity to be in stores, and all that other stuff, and Waid would argue that he can't claim damages because he actually ended up in a better position than he would have if Waid had not interfered. Waid can't basically take advantage of his own wrongdoing in that way.
However, that might eliminate incidental or consequential damages that aren't directly related to the contract violation itself, if Meyer didn't suffer those damages.
However, he clearly suffered the direct damages of having to self-publish, and the right to contract with a party of his choosing who had already chosen to contract with him as well.
The damages are not a monetary amount.
They are if you want to collect them, or more than $1 nominal damages. But the direct damages here are pretty easy to establish if you can establish there was actual illegal tortious interference. Proving that is the most difficult part. Waid could and certainly will argue he was entitled under his First Amendment rights to call up Antarctic and tell them to cancel their contract with Meyer, or just in general to badmouth him. His status as an actual market participant weakens that. He wasn't just criticizing someone he didn't like, he was engaging in commercial speech to knock off a direct competitor who would otherwise have had his books on the same shelves as Waid's own. He didn't want to inform them of something for their own benefit. He wanted them to illegally cancel a contract they had already made, that he knew they had already made, and he didn't give a fuck if it was illegal for them to do that. In fact, if they didn't do that, there would be a price to pay. He made that very clear.