What kingsroadgal said - there's a sticky on the BA forum and I think the costs will be in there.

However, bear in mind that World Traveller Plus only has something like 24 seats in it - the chance of 5 not having been sold seems slim and even then you're likely not to end up sitting together if you wait until the airport.

Also, these upgrades never seem to be offered out of Heathrow, only for inbound flights.