It takes a special kind of person to be happy with a cheap thing they paid way too much for.
We (as a species) will go cognitive miles to avoid actually facing embarrassment or admitting to being fooled. Your thing might not be overpaying for a diamond and it’s unlikely you can see your own instance(s) but I’m sure you have seen it in others on the regular.
Though, as a man, figuring out your partner's expectations of rings and weddings (and family and children and ...) is part of the dating process. You really shouldn't be blindsided at the engagement phase.
I'm reminded of a friend from high school who is got divorced after finding out his wife of two years didn't want and never wanted children. His defense was "it just never came up!"