I have a similiar situation. I live in Toronto (GTA), and go to university in Waterloo, about 150 - 170km. So during the week I'm here in Waterloo, but I like to come home on most weekends. This costs me around $30 round trip, thats around 90 bucks if I go 3 times a month. I make around $50 - $100 a month selling computers, and this usually doesnt cover it. Sometimes my mom will pay if she really wants to see me
So far I've never been stranded here, but its just a matter of time before I have a bad month, and run out of gas $.

The most improtant thing is to buy gas on monday and tuesday, I buy 87 octane, and save 6 cents a litre (I usually buy 50 litres, so it becomes sgnificant) if I buy mon/tues vs fri/sat/sun.

Since your trips are frequent, but fairly long, a diesel would suit you great.

And if you dont mind me asking, what school do you go to?