One commonly used figure for how much one should spend on a ring, invented and promoted by the sellers of engagement rings, is two months income. I am NOT saying that you need to spend two months income on a ring, but if you anticipate two months income being significantly less than the value of the “ideal” ring, the “ideal” ring is too expensive.
Talk to her about Big Picture Finances–will spending 18 K on a ring make it harder to put a downpayment on a house? Buy a new car? Pay off your student loans?
I’d be uncomfortable knowing that my fiance/husband was paying off loans on my engagement ring. Then again, I wouldn’t demand/suggest/covet a ring worth 18K.
(Also, although I hope you’ll love her forever, and vice versa, have you looked at the divorce rate lately? Do you want to be paying for a ring for a woman you no longer like, let alone love?)
Another option is the buy her a ring that you can afford now, and suggest that maybe you can upgrade down the road ten years, or twenty or something.
Talk to her about what she most likes about the ring you mention. The Tiffany Name? Do they have something with a smaller diamond she could like? The Setting? Does a similar setting exist from a less pricy designer? The size of the diamond? Could she settle for Cubic Zirconia? (I wouldn’t, and I’d be pissed if you suggested it. But that’s me. I’d settle for a gold or silver engraved band that wasn’t wedding-y a whole lot sooner than I’d settle for a fake stone. But you aren’t buying this ring for me.)
18 K is a whole lot of money for a engagement ring, regardless of your future earning potential. And sadly, the mark-ups on jewelry are such that if someday want to sell the ring to get some cash, the cash you get will be far, far south of 18 K. So, either your lady has no clue about the value of money, has eyes bigger than her/your budget, or is someone so greedy you’d be better off splitting up now. I’d assume cluelessness rather than greed, but I still think you need to talk finances before you do any major ring shopping.