I told my husband (then boyfriend) I didn't want a ring, I just wanted to be married to him.
He thought I was being romantic, or something, and bought me a ring anyways.
First, he didn't spend an obscene amount - probably $1,200 (which still seems high, but not in comparison to some others I've heard I've). Second, if you pick the right style, cut, etc., you can give the appearance of a bigger diamond, when you're only paying for a .75 karat stone.
Third, I wasn't kidding when I said I didn't want an engagement ring. Don't get me wrong, it's beautiful and I do like how it looks on, and I did like the attention random strangers (usually old ladies) would give me when they realized I was engaged to be married.
But honestly, I often wear only my wedding band now that we are married. (We took our parents' bands, so we didn't pay anything extra for them, and, mine is a sweet 70's retro style that looks really cool with my more timeless and simple engagement ring).
The reasons I wear only my wedding band: I'm uncomfortable wearing a single item of jewelry that costs more than I have in my savings account. (I've since fixed the low savings balance, by the way)
And, as a result, I'm worried I will somehow damage it, lose it, or get it stolen.
Also, it doesn't fit well into my motorcycle gloves.
So, in summary, consider whether she'll actually feel comfortable wearing the ring in addition to the costs.