In September I will start my master at Brussels. My boyfriend is a student in Leuven, and we could rent an apartment together. In this way, I could save money from the renting fee, but I would have to pay for frequent journeys Leuven - Brussels (I estimate 4 round trips per week). Also, we have been in Belgium for a few days, and it seems that life in Brussels costs more than in Leuven. Another option would be to rent a single room in Brussels. Which is the less expensive option in your opinion?
In terms of money living in Leuven will be cheaper - a month ticket Leuven-Brussel will cost no more than 150€. I doubt that you can find such a low-priced room in Brussel.
Cost of life is clearly cheaper in Leuven, not only for rents but also for food and leisure. If you aren't concerned by the time lost in transports, then you'll probably want to live in Leuven. You also have areas in Brussels that aren't too expensive like Molenbeek.