I think the psychology behind it is simple.
She thought it was excessive , considering her feelings for you.
Maybe she had already decided to end it- who knows?
But to spare your feelings, she said she'd wear it.
I'd say more context is needed.
You say 'expensive' - that for some men could be £50 or £100 if they are skint, or it could be £5K or even £50K.
If you spent more than you could afford, and it was a really big deal, then she would know that and should have returned it to you.
If it was a token gesture, then she's just about ok keeping it.
But I can't think of many women who would keep it as a reminder of a relationship they ended, unless the planned to sell it and keep the cash. Or they had no moral scruples.