I like the Roissy response that Eternity gave. Sounds like something similar to what I would say.
It's been awhile since a girl has flat out asked me to buy her a drink, but I think I remember using this one a couple of times:
Her: "Are you going to buy me a drink?"
Me: "I'm sure every other woman in here would like me to buy them a drink as well. Can you convince me as to what makes you the special woman that will get me to do so?"
If she's intrigued by your response and seems interested, she might begin qualifying herself and then flirting with you (as a way of "seducing" you into buying her a drink). But this can totally work in your favor. At this point, you just begin flirting with her sexually too, and if all goes well, the conversation just might end with something along the lines of:
Me: "I won't waste your time with the drinks they serve here. I make the finest Don Julio margaritas at my 'personal' bar. Come with me and I'll make one for you."
All of this is said playfully and sexually of course.
- Franco