If we are being really technical about things, at one point not too long ago an extra box used to cost £9.50 (before that it used to be £15). And if you had more than one V+HD box on M/M+ or L there would be the £5 per box as well
Then Virgin reduced it to £6.50 if you had an HD box activated (excluding any V+HD fee)
Even with an extra TiVo box costing £9.50 a month, it's still cheaper than Sky and you get a lot more for the money