A real friend, IMO, would be understanding. After so many invites, she would have to expect people to decline. Even if she is looking to make some additional cash, she can't expect it to always come from her friends. If you people relied on friends for monetary support like that, I'd either be flat broke or swimmin in the dough!

Be honest with your friend. Explain that you only budget so much for miscellaneous expenses every quarter or so and that it's just not in the budget.

Good luck!