There already are $20K 4 seat electric cars (after subsidies) that go 150km, and charge up overnight on a 120V connection, and they're a niche item. 300km would be a definite improvement, but I'm not sure it would change the world overnight.
150km represents something like a 90 minute round-trip commute at highway speeds. 20% of Americans have commutes of this distance or greater, and if they don't have a charging capability at work, these vehicles are non-starters. I'm one of these people - my commute is on the ragged edge of performance range of electrics right now and I can't risk it. I'd buy a $20k electric with a 300km range in a heartbeat.
Electric cars are a very tight niche now. If you have a short commute, it hardly matters whether you drive gas or electric because the energy costs are cheap either way, and gas vehicles are still cheaper to buy. If you have a long commute, you're going gas, because electric can't get you the range. The only place electric cars work is in the extreme ragged edge of battery range, in places where charging at both ends of the commute is possible, and where gas prices might come close to offsetting the cost differential in the vehicle purchase.
Get the range into the regime where range anxiety is a non-issue for 99% of car owners at this price point and electrics will take off. Right now 20% of car buyers know they need more range, and probably another 20-30% aren't sure that the real range (not the quoted range) will meet their needs. 300km range would do this.