Most contracts are multi-part documents. Frequently there are a number of terms that are part of the contract that one might object to, not just the compensation clause.
If you reply that the contract terms are not agreeable or are unacceptable and they wish to continue the negotiation, they will have to write back to find out which specific terms are disagreeable to you, prompting another exchange of messages.
You'll have to speak specifically about compensation during the next round. Why not get 'er done now?
I like this phrase:
"I'm not willing to do the work for the compensation offered in this contract".
Also once you've said that, you should make a counteroffer to propose an amount that you would consider doing the work for. Any reasons you have to justify why your proposal would be a fair price, include in your reply.