It's like this, if you can't afford an $8,000 ring, then so be it. How can anyone justify spending $8000 when their gross income is $40,000. They, jewelry and diamong people, are laying the same rules for a rich person down for a middle income to low income person. Rich people can drop that much money because it doesn't affect their ability to live their lifestyle.
You can always tell her if you earn more in the future, you can always get something bigger down the road. If she dumps you now, then fine.
If you are going to play keep up with the Jones now, it's going to continue after the wedding. Gotta have a bigger house, nicer car, better neighborhood, nice vacations, it's like they want the NBA/NFL/MLB wife lifestyle. And when you go broke, the whole 'have and to hold for richer or poorer' is flushed down the toilet. They are going to leave you with your debt.