There should be a prepaid plan with some data. Like for the $35/month Base plan you should get like 250MB of data to use in that 30 days. That is one big thing that Virgin has on prepaid that I would switch for.
I'm not sure I understand your request.. what you are asking sounds a LOT more expensive than the data booster. That costs $30 but it carries over... making 250MB cost only $7.50.
$15 (base plan) + $7.50 = $22.50, which is consideraby less than the $35 you're requesting! And, as I said, the data you don't use carries over to the following month!
What I'd like to see is a slightly cheaper baseplan... but considering how the rest of the prices have gone up this past week, I don't hold my breath... ;)