I wouldn't ask my therapist. I wouldn't see it as my therapist's problem to help with my use of money? I would be inclined to go with my original plan and not see her on Monday but maybe e-mail a bit more than usual that day since it's the holiday week.

I would call her, tell her you cannot afford Monday so have to cancel and see if she offers anything in return like a reduced price session, etc. But I don't think she could "morally" give you a free session as that would break the frame of therapy; if then, why not next month when you have another/different problem, etc.?