By taxing the wealthy. So...what's your point? There used to be a day when going to a Public College or University cost next to nothing. Now? The average student loan debt from attending a Public college is $30K. That's disgusting. It also harms the economy because you have new graduates who delay things like starting a family, buying a house, even moving out of their parents' because of the debt they incurred going to public school. Those loans also show up on credit reports, which employers use now in the hiring process. We need a free public college system in the US, for nothing more than to remain competitive with the rest of the world.
Well, single-payer would just be an expansion of Medicare and would be very simple to do...just remove the disability/age requirement and inch payroll taxes up slightly. It's more than made up from the savings from no longer having to pay outrageous premiums, deductibles, and drug costs.
Sanders proposed a 6.2% tax on workers and businesses, replacing the Medicare tax and eliminating Obamacare, it's subsidies, and Medicaid. So by expanding Medicare, you are cutting out Obamacare and Medicaid. So you're trading two (or more if throw in S-Chip) programs for one. The average worker spends about $5K a year on their employer-provided care with the employer paying about $12K. So in total, it costs $17K a year to insure one worker through an employer-provided plan. To pay that much under Sanders' plan would require the worker to have a wage of at least $80K a year (Median income is about $53K a year), or a business of 50 employees to make about $10M a year in profit. Very few firms of 50 employees or less even come close to that, and those that do are mostly in Financial Services (who don't produce anything).