It's not about ring, not about diamond, it's about the value

Not $ value, but how the boy values the lady.

订婚戒should in general be 2 months of boy's salary.

是一个重视和仪式感的衡量

Why 2 months? Because it's serious enough, and it's quite achievable. 

If a boy can not save up 2 month's of salary, the future would be worrisome too.