I have just looked and to upgrade to a v6 box will cost me an extra £59.13 per month. I have only been with virgin for 2 months and I am on the premium package.
The costs are detailed here.
You need 100Mb internet and if you are on a lower speed, have to upgrade to a higher package. Costs on the link are one off costs, not monthly.
However if you are out of contract, you may be eligible to receive a loyalty discount providing you sign up for another 12 month contract.
Your problem is just that. You've been with VM for two months. Therefore you lose any loyalty discount you had when you joined. Ring retentions to see if they can offer a better deal but don't get your hopes up