It doesn't cost more than the current system per this article which you can find on this thing you're currently using called the internet http://thehill.com/policy/healthcare/399625-sanders-thanks-koch-brothers-for-accidentally-making-argument-for-medicare
The economy has not "taken a turn for the better". It's been on this track for a couple of years. That "4% GDP growth" being touted is a short-term benefit of the tax cuts, just like every tax cut prior. Reagan implemented the largest tax cut in history, the economy took a shit, and then he had to implement the largest tax increase in history to fix it. Duhbya had 2 tax cuts and 2 wars coupled with an unregulated banking system that caused the financial collapse. For the last 38 years, Republicans have proved their policies don't work or only work for the super rich. You have Google. All of this is supported with hard evidence so educate yourself.