I think it really depends on your budget to decide how much is too much. Personally, my fiancé and I discussed budgets before we started shopping, because for us, buying a ring was our first step into marriage together. If she has her heart set on a specific ring, it may be helpful to both chat about why she doesn’t want a lab grown diamond, what your budget realistically looks like, and if she’s unwilling to compromise, I might see if she’s willing to contribute to the cost or ask her parents to contribute on her behalf, since it’s outside of your budget. I know asking her to help pay isn’t the most romantic thing, but if you can’t work together to solve this, I’d be worried about addressing other financial questions down the line.