Right now, EV owners are using home chargers. I'm sure there are very smart people who are aware of potential problems. There is a plan to lay a magnetic induction grid under one mile of street in Detroit. There is no need to harden anything, just maintain a high level of security, with manual backups in place. Penalties for so-called hacking attempts should be severe. At the least, being barred from internet access.
Cost? The U.S. government has no money of its own. The taxpayer is footing the bill. Rest assured, you will be charged a hefty price at an official 'charging station' as opposed to charging at home.