Re: Cost is relative
The subsidy costs money. Tax money. It should be targeted to where it does the most good. Gives the best incentive. Paying it to allow rich people to buy a Tesla as a second car to play around in when they aren't in their giant Merc is not a good policy.
Set the subsidy low. More of it to spread round where it's needed. Incentive for car manufacturers to produce cheaper alternatives. Win/Win.