It's apples and oranges. You either get a lot of stuff for cheap with no rollover and a shorter expiry (almost all monthly plans) or you pay more per unit/minute/text/meg but get to use them over a longer period of time.
Net10 and Tracfone have VM's 20 cent a minute default rate beat cold.
And their Net10 Unlimited and Straight Talk offerings give unlimited data/text at prices that rival or beat VM's unlimited plan.
The industry prepaid trend is to get people on monthly plans with anytime minutes, no nights/weekends/m2m and no rollover.
That way they have a nice steady income every 30 days. With no leftovers.
VM is weak in this area.