<human>: Could you distill the essence of the text into 1-3 sentences?

My girlfriend and I have been dating for four years and living together for two. Our relationship has never been better and I feel like we're ready to take the next step (similar life goals/aspirations, financially secure, enjoy each others company, etc). 

So recently I've begun looking at engagement rings. I'm not a particularly vain person and I have little interest in contributing to the idiocy that is the global artificial diamond monopoly (thanks de beers). Nonetheless, I've accepted the fact that I will have to purchase one such worthless piece of carbon because my darling future fiance has been brainwashed along with the rest of the women on this planet.

The question I'm struggling with is how much money is reasonable to spend on said rock. She has made it clear that she wants/expects a 2 carat stone, despite the utterly preposterous $15000-20000+? cost (she seems unable to debate this logically). Does anyone have advice on how much these things should cost? How big is 'big enough'? 1 carat, 1.5, 2? What is considered reasonable in the 21st century? Is she crazy? Am I crazy?

For reference, I make six figures but live in an expensive city, and save most of my money rather than spend it.
<bot>: Girlfriend wants an expensive engagement ring that I can technically afford but feel would be a long-term financial catastrophe, need advice.