I agree with your mother. Unless you are going to be under severe financial stress, I think that the money aspect of you decision should be the least significant factor. I know that I don't share the same perspective though. I personally choose the school that offered the least amount of money per year that also had the most expensive average cost of living. But that doesn't matter much to me! Leaving meagerly for the few years that you are pursuing your studies--the time in which you are supposed to submerge yourself with your studies--is a very mild compromise.
On the other hand, I would consider your girlfriend as a more serious issue. Is it a serious relationship? Being separated like that for 4 years or so can be very intense. My wife is currently in Germany, so I understand how stressful it can be trying to maintain a long distance relationship. Luckily, we only had to manage for 2.5 years (1.5 down, 1 more to go), as opposed to, say, 4 years to earn a PhD.
Best of luck
Chris