I mean it’s all opinions of people who don’t know you and your fiancé, but get some things she’d like in a ring, like the stone shape, material of ring and maybe a design cue. Then what I did is spoke to some friends on where they had liked to buy jewelry from locally and found a family owned custom jewelry shop. I found a few things I liked, took her and she picked out things and the shop helped me compare and create the two things into a ring I thought she’d like.
Honestly it won’t matter on cost (unless the person cares) but I did sell a pin to help fun the purchase. I think I spent around 5-8k? For the engagement and wedding band together. And if she wants to change or add a band in the future that’s totally great. But step one to any good relationship is communication, step one to getting a ring is communication :-p