Engagement and wedding rings are gifts. They are separate property of the recipient, and pretty easy to prove as such.
This answer DOES NOT establish an attorney-client relationship. This answer is based on the limited information provided and is not intended to be conclusive advice. There are likely other factors that might influence or change the advice after a more lengthy consultation.
The answer depends on whether you are married to "your soon to be ex" or not. If you are merely engaged, the ring is considered a conditional gift, meaning that it was given to you on the condition that you marry the man who gave it to you. If you have decided not to marry him, then you should return the ring. On the other hand, if you have already married this person, then the ring is a complete gift and thus your separate property. In that instance, you do not need to return the ring and he has no basis for suing you for the cost of the ring.