I think you are misinformed about the subsidy. It is $7,500, not $75,000. But I agree that cars that cost over $50k like the Tesla Model S should not get the subsidy. That is a luxury car and their customers are not buying it for economic efficiency. Keep the subsidy for the $35k Chevy Volt and the $30k Nissan Leaf to bring them down to $27.5k and $22.5k, respectively.
And no, you don't need to double or triple the subsidy because all you have to do is make them price competitive with gasoline cars, not make them free. Over time, the subsidies go away (each manufacturer gets 200,000 before they ramp down) so they are only there to help offset the price while the technology is new and expensive. Prices have already fallen by a few thousand dollars in the four years they have been on the market and they will continue to fall as li-ion batteries continue their downward cost trend.
I don't believe in any subsidies right now. The R&D of things like batteries is important so we should give money to national research labs or places like that right now. Economy of scale for batteries is a joke. We are using up lithium, not scaling up. The more we waste on overkill like the Tesla, the more expensive it will get.