Good question. I enjoy being Platinum, but everyone is different because what everyone values when flying is different. I fly Virgin about 35 times per year and so I'd value the things I get for being Platinum (as opposed to Gold) as something like the following:
Free internet: $5-$10 per flight.
Extra Velocity points: $5 per flight
Free EconomyX: $10-$15 per flight
Fly Ahead: More difficult to price because I don't use it every flight, but when I do it's fantastic. Maybe $200-$300/year?
So "break even" for me, is probably around the $600-$700 mark, but in practice some of the above (eg. internet, extra legroom) are "nice to have" rather than "need to have". So realistically I wouldn't want to be paying that much.