as above, fare code is the main factor
but be careful for tickets that were not purchased direct from VS as they can be zero value when it comes to cash upgrades - i've been quoted prices for airport upgrades that were higher than simpy buying a new ticket - also fewer fare codes are miles upgradeable
i don't think light loads will cause VS to drop cash upgrade prices, but it greatly increases the chance of being able to do a miles upgrade at the airport
if you're not in flying club,
join immediately as at least you'll earn some on the first flight
if you do have FC miles, is it enough to get an upgrade? rate depends on route and cabin, you'll still have to pay a bit for "taxes and surcharges"
check if you've any hotel/store/card/other points you could transfer at a reasonable 'exchange rate', see...
http://www.virgin-atlantic.com/gb/en/fl ... miles.htmlthere's a 25% bonus on buying FC miles at the moment, if you are a bit short then topping up may be good value