How to deal with cheap / frugal customers?

I have a small shuttle service, we offer private transportation to and from the airport, we aren't a shared shuttle, it is a private service and we use a large SUV. We charge way less than a taxi. Anyway, every so often I get people trying to bargain our rate. I had one lady ask for $20 off! So I offered to take $10 off, but nope that didn't make her happy enough. Anyway I don't want to ramble but it really frustrates me because gas is expensive and not to mention the fact that we are already CHEAPER and cleaner than a cab! What more do people want! and it seems like when I even offer to give them a discount it still doesn't make them happy! Maybe I shouldn't even offer them a discount at all? is it better to not even play their bargaining game?