The current cost for covering 45 million Medicare enrollees (including patients' portion) is estimated to be $484 billion in 2009. Since enrollees pay a 20% co-pay for most services, the total government-payout will be 80% of that total, or $387 billion for 2009. (This is consistent with the Treasury Department's $390 billion figure for Medicare--
http://www.kff.org/medicare/upload/7305-04-2.pdf --for fiscal year 2008 in Table 9.) This total includes Parts A, B, C, & D.
If 45 million uninsured Americans were instantly given Medicare-type insurance, with 80% government funding, it would cost taxpayers no more than $387 billion, since the new patients would be younger and healthier. But $387 billion is a gross overestimation of the predicted cost, given the much smaller costs of covering healthier, under-65 year-old patients (as opposed to Medicare's sicker, predominantly over-65 patients).
Therefore even if the government were to pay the 80% share for a Medicare-type program, the costs would be nowhere near the $300 billion to $1,200 billion estimated by the government, which are based exclusively on the extensive subsidization of private insurance company premiums. The amount would be further reduced if there were extra premiums paid by those who voluntarily buy in.
According to the above estimates, it would be 3 to 6 times more expensive to taxpayers to enroll the uninsured in private insurance plans than it would be to extend Medicare to cover them, even if taxpayers paid entirely for this public option. Not only would the proposed amendment be a lot cheaper, it could be implemented immediately.