I think that the answer to this question is simple. If you really enjoyed the date, you are interested in her, she has invested in you and you can afford it, you can pay. But it should just be a polite way to say thank you for the wonderful time I had with you (and your investment in me) and not expect anything in return. If on the other hand you didn't have a good time then you shouldnt pay. It's like saying thank you for nothing.