I sort of got it drilled into my head by my mum that if you let a guy pay for you YOU'RE PRACTICALLY A PROSTITUTE. I mean, I think that's kind of harsh in a lot of situations (e.g. he asked you out, you're broke and he's rich, the second of those examples my mum was actually in once and she still insisted on paying her own way) but there are indeed some girls I've known who are simply outraged if the boys they date don't buy them lots of stuff, which I feel reflects kind of badly on my gender as a whole and shames us.
So I try really hard to pay my own way. My boyfriend does have a job (albeit not the best paid one but then he lives with his parents so has zero outgoings) while I'm a poor student. So I'll accept a treat every now and then, particularly if we've gone somewhere that I've only agreed to because I know he'd enjoy it (somewhere with his mates, usually). But it's been about a 45/55 balance of who pays. Sometimes I'll pay for a whole meal, sometimes he'll pay for a whole meal (it's easier than paying separately, but if it's a first date you can't be sure they'll be able to pick up the check the next time!)
EDIT: Speaking of who-asked-who-out on dates, women who aren't afraid of breaking the tradition of 'the men are supposed to ask!' tend to be women who also aren't afraid of breaking the tradition of 'the men are supposed to pay!'
Not always, though, of course.