Have you been weighing the idea of buying a second hand 2 carat diamond ring for your fiancée, as opposed to a new one? The first is insanely cheaper and almost flawless when you look at it, so why wouldn’t it be good enough to propose with? Did someone tell you it brings bad luck? Or do you think your sweetheart deserves something new on her finger to mark getting engaged?
Do not knock the idea until you consider it in detail; all that sets a second hand diamond ring apart is that someone owned it before you. Assuming you are able to look past that and pragmatically focus on acquiring the perfect design for the perfect price, this is not very different from getting an heirloom ring to propose with.
Besides, a partner who appreciates the financial wisdom in this is certainly a keeper. From your side, being 100% honest at the beginning is something she will remember later on, and in time come to love you more dearly. This kind of understanding can easily keep any “bad luck” at bay, because each of you would be watching out for the other.