Do what works with your budget and a stone with significance. There is no hard and fast rule to buying an engagement ring, so don't sweat over forking over two months' salary if the both of you can't afford it. Keep in mind that you have more wedding expenses to battle. Focus more on the significance of the ring instead of the price tag. Perhaps she'll be happier with an engagement ring that has her birthstone, or maybe she wants a man-made gem or a preworn ring with history. There are plenty of wallet-friendly options out there, so don't limit yourself to the never-been-worn traditional diamond ring!