Whatever you’re comfortable with. Forget the “3–4 months salary” bit.
But… that said… realize that your fiancé will be comparing her new rock to her girlfriends’, her mother’s, random folks in the checkout line, etc.
So, do your best and seek out a good value (the “four Cs”: carat, clarity, color, and cut). In that regard, shop around, and try shops where they make their own.
And, this is important, she knows what shape rock she wants (round, princess, emerald, oval, heart, etc.)... you better find out what it is!
Best of luck!