It’s interesting you bring this up. This is actually something we are working on right now.
A while back we tried to set it up so that if the price in a product was simply empty, it would still allow offers. Unfortunately, the necessary hook we required was not available in WooCommerce at the time, and this got put on hold.
That hook now does exist, so we just yesterday made the change in our code to allow this, however, it causes the Add to Cart button to show up along with the Offer button, which is not what we want in that case.
The problem is that the Offer button gets displayed using the woocommerce_after_add_to_cart_button hook, so if the Add to Cart button is not displayed, then our offer button goes away, too.
We are actively working on a solution to this issue and hope to have it resolved within a day or two. It will then be included in our next update.
As soon as we do get it worked out and merge the changes to our code I will update here so that you can grab it early if you would like to.
