without knowing what your grandmothers car is, I can kind of see your moms point. She is paying for a PT Cruiser, a very flashy car. While I don't like this type of car, if I was paying for a flashy car... when I went out with the guys, I would want to use it.
I've been looking at cars lately and I have found that there are several cheap cars that might suit your needs.
Check this thread:
http://www.tfproject.org/tfp/showthr...threadid=40416
It might help you find a beater that will give you some added freedom.