Well... also depends on your travel pattern and frequency.

If you are a frequent Y traveler, it worth quite a lot (for F lounge access and op-up opportunity, priority seat selection, extra luggage). Nevertheless, I will still place a valuation of less that US$1,000 (unless you are "water-good" carrier, then the extra luggage really worths something).

If you are a F traveler / occasional traveler, it doesn't worth much.