Since there wouldn't be much cost to individuals, there would be an increase in demand. In other words, people who go without drugs because of their cost would become more willing to take the drugs they need.
This would be offset by the economies of scale associated with group purchasing. We make this estimate on the assumption that the federal government, or a government that would be a payer, could negotiate savings or rebates based on the savings already achieved by certain group purchases. In addition, this government would benefit from rebates that are more or less secret or confidential, but which we have been able to estimate at around $2 billion, according to our sources. That's where the savings you cited come from, which can therefore be attributed to group purchasing and the significant negotiating power of a single payer.