Question About Prices

Hi everyone,

I have been talking with my girlfriend about engagement rings but I'm so lost when it comes to price. We are both women. I'm not really into wearing rings so I said if she ever wants to get me a ring I just want a simple band.
However, I think for her she wants like a proper engagement ring and she wants one that she says fuses to her wedding band I think? (Apologies if this is not even possible. I'm not very knowledgeable about jewelry) I just want to know if a budget of 15-20k is going to get her a shiny, big diamond? She does not want lab grown and wants a gold band I know that but when I look at Tiffany's it's like 85,000 for an engagement ring after you select a diamond and others are like 25-30,000. Is my budget too low? I will pay whatever she wants for her ring but I was just wondering if you all could help me figure out like what size diamond is good? Or if I'm being too reserved with my budget?

Thank you for your time and sorry for sort of rambling. I'm really bad when it comes to figuring out jewelry.