This is a toughy...it's highly situational, really. In the example of the awesome concert versus her birthday, i'd go for her birthday. It's my money, so it's mine to waste. My friends can give my ticket to some1 else. If they bought the ticket for me ill be happy to pay m back.
Good friends understand that sometimes the girl comes first. I can guarantee you that my own friends would respect any girl-related decision i make. They may try to give me advice if they think i need it, or they believe i'm making a bad call (like a girl who ends up mentally killing me, be it in- or adventertendly), but they know that sometimes one needs to burn his hands to know how it feels.
A good girl understands and respects your decision to spend time with your friends every now and then. She might not always be happy about it, but she has friends too, and she'd want you to let her go out with them aswell.
Striking the balance is hard, and as i said before, highly situational. Give me an example and i'll be glad to tell you what i'd do in such a situation, and i'll even try to come up with some scenario's of what might happen when stuff is handled in that and other ways.
Disclaimer: I don't know how many women are on the escapist, but where i'm saying "good girl" you should read "good man" aswell. Respect goes both ways.