I'm afraid this comes up fairly frequently. Virgin Media's charges for use in the USA (and elsewhere outside Europe) are extortionate. It's too late now, but for future trips you may care to buy a temporary PAYG SIM from the Three network. They provide coverage in the USA and some other non-European countries at normal UK rates, and one of their PAYG SIMs with 12 GB of data to cover a month's trip costs £20.
For your current trip, if you can find a suitable shop, you could buy a pre-paid SIM card from AT&T or T-Mobile, as long as your phone is unlocked. This will charge (reasonable) US-domestic rates for calls and data. If you do this just make sure that the SIM works on GSM.