Well, you can structure the contract in any way you like. But there's no guarantee that the client will accept it.
It's certainly reasonable to impose a cancellation fee. The only question is whether 50% is the right figure. I can't help you with that. The "right figure" is one that is high enough to cover your losses, but not so high that the client will refuse to accept it.
You've also got to make it clear what the 50% is based on. To charge "50% of what my total pay would have been" seems a little vague. You'll need to be more precise in the wording of the contact.
Also, keep in mind the possibility of the client cancelling before the contract is signed, that is, the client deciding not to go ahead with the contract after all. In that case, you won't be able to enforce the cancellation fee. So better not give up your day job until everything is completely signed.
Finally, I'd advise you to discuss the issue with the client before presenting him with the contract. Put your cards on the table, explain the situation, and make sure you have a meeting of minds on the issue.