One question on how much to spend is whether a woman values/likes/wears jewelry at all. My mom is really into jewelry--real stuff--nothing fake--and when I was younger, she gave me some very nice rings and necklaces. I bought some diamond/ruby earrings when I was 11 with my allowance. My tastes were generally conservative--small stuff, nothing flashy, but my mother always said "When you get older, you'll wear bigger stuff."
When I went to college, I lived in a big city. I'd regularly pass beggars/homeless, and I didn't want to be funding a drinking habit, so I would politely decline to give them anything. But I would feel horribly guilty if I was wearing any jewelry--I just felt like I was just slapping them in the face "hey, I'm willing to wear some useless expensive rock around my neck, but I can't be bothered to help you get lunch" so I stopped wearing jewelry altogether.
After my senior year in college, most of the jewelry was stolen from my parent's house, and I used the insurance money to buy a laptop. I've never regained any comfort wearing anything other than my wedding ring except for weddings and fancy dinner parties. My mom occasionally gives me jewelry, but I never wear it. It's kind of a bone of contention with her--she wants me to inherit her jewelry, but she's afraid that when she dies, I'll sell everything (which she believes would be at a loss), so she's threatened to give it to someone else (probably my daughter). But there's no way I'd ever feel comfortable wearing her stuff.
So if you are marrying a woman who values, wears, and loves jewelry, then it probably makes sense to get her something "expensive." But if you have someone who would feel uncomfortable walking around with a 2 karat diamond ring, then get her something small.
But be aware, that whatever you did this time, will probably continue long-term. A woman who likes to get gifts of expensive jewelry isn't going to stop liking it after marriage.