I think 15K is a big budget for a ring, and 30K is over the top unless you happen to be rolling in money. Your mother is definitely projecting. Ignore her, and convince your fiance to do the same. It's likely she'll do this with everything you buy as a couple, so learning to tune her out, and realizing now it's just her own unfulfilled wishes being pushed onto you, will be essential to your sanity.
I totally agree with this and everyone else's comments. Sounds like my MIL to be honest, and yes tuning out is sometimes the only option.... She just doesn't get it even if I explain my views. Not worth it, makes me agitated!