What if you said something like 'Yep I'm single..but I can be yours for $300 an hour.' and give her a grin and a wink?
How'd that work out?

What about

Her : Do you have a girlfriend
You : Sure, whenever I want (smile)

Not sure about the outcome of this one cos it might come over as cocky, but I'll fieldtest it

I pretty much rely on cocky funny stuff as a regular part of my game, but depending on what type of character you've built with the girl you can try this. It works for me cause I say stuff like this all the time. (I read this thread somewhere, not sure where at the moment)

Her: Do you have a girlfriend?

Me: Yeah, I actually have 9 girlfriends (brief pause and smile, but dont laugh) but I've been looking for lucky girl #10 (then look at her as if she may be the one)

Me: Being my girlfriend is actually pretty tough, they coordinate together to make my lunch, iron my clothes, take me out you know wine me and dine me. you probably couldn't handle it (laugh roll off into something else)