However, despite these legal changes, it's not correct to say that the ring doesn't cost the proposer anything.

You've changed my mind: there is a real cost to the ring. I considered the ring a thing equal in value to its price but didn't think it through enough to realize that after it's bought it only retains much value (as sentimental value to the couple) if the proposal succeeds. Thanks for the links; I had no idea diamonds were so over-priced.