It says "predicted" costs and based on the ability to still use federal funds. A 15% payroll tax would be needed to fund it. Eliminating private insurance takes out a lot of clerks from providers and insurers-unemployment claims and retraining would be just one of the issues with the old system along with redoing the computers systems & administrative record keeping. More state level government employees would need to be hired and budgeted for as well.
Californians are already complaining about a additional gas & vehicle tax of about 500$ a year. It tripped a recall election of a politician who helped push it through. No way would a 15% payroll tax fly.