Ok here are the best two answers field tested and mother approved:

If they are bitchy about it and you know it because they just have COCK TEASE written all over them you have one response to win them over.

HB10 - Hey why don't you buy us some shots (drinks).
You - Sounds good I'll get you girl a CUM SHOT - how many do you want

Done well you should get a good response. The goal is to make it sound like an actual shot at first too...then if they are dumb as fuck tell them they are only available in the bathroom...they don't serve it at the bar

The second is:

HB7 - Hey will you buy us drinks.
You - I'll buy you a drink if you can tell me something other than love that money can't buy.
HB7- Almost always respond with Happiness
You - Well I'm not going to buy you a drink if anything you should buy me a drink
hb7- why??
You - Cause if I BUY you a drink you won't be HAPPY!!!

p.s. no matter what she says you can 99.99% of the time buy it with money (even love)

if anyone has ever gotten a response that you can't actually buy I'd like to hear it. The closest a girl came was when she said SUNSHINE. I almost wanted to smack her cause I though butterflies were going to be the next words out of her mouth. Turns out you can't buy sunshine...I ended up liking the girl and she proved to smarter than anticipated. So if you get this one girl..BUY her that drink it'll be WORTH it

Lol! I love this!!! Brilliant work!