Don't kill me, I'm new!!

I recently booked a holiday with a travel agent to NYC in the coming summer.

As part of the booking I'd be flying with VS in Economy from LHR to JFK in the "O" fare bucket.

Unfortunately I wasn't offered any option other than Economy with the agent and was advised that I'd have to deal with Virgin for an upgrade as opposed to them.

When approaching Virgin, I was told that I'd have to arrange an upgrade on the day at the airport as opposed to online.

I just wanted to know if anyone on FT would know typically how much an upgrade would generally cost at the airport and whether this is all that likely.