Jinkster -
Two days before the flight, through the Virgin website my colleague was quoted £350 return for an upgrade from Economy to Premium Economy.
On the telephone, I was quoted £420. I decided to wait until the airport.
At the airport I was told that there is a flat rate of £300 each way for the upgrade.
So - to answer your original query - it's worth doing your homework before you get to the airport. It may even be cheaper to book a restricted premium economy seat in the first place if you don't need flexibility.
BYW, I stayed in Economy but paid an additional £75 to secure a window seat on an exit row. The extra leg room was well worth it.
Hope this helps.
Essouira