This doesn't work. I have tried it. She (my GF) just argues with me about any purchase I try to make that is around the ballpark of what she is expecting me to pay for a ring.
e.g.
Her: So you want a new MacBook Pro, but you can't afford a $3000 engagement ring?
Me: No, I need a MacBook Pro for work.
This has been going on for quite some time now.