If you're willing to sacrifice speed and comfort to travel cheaper, then car pooling or an advance booking on a Eurolines bus is likely to come out cheaper than the train.
From Zürich to Brussels, you have to go through either France or Germany. It's quite possible that careful shopping between SBB, SNCF, DB and SNCB will yield a cheaper price than booking a through ticket from a single operator.
If you look at a railway map, you'll see that your journey is highly likely to take you through Basel, and then you have many possible routes either via Germany (with Karlsruhe, Cologne and Aachen being likely way points) or via France (Paris being a fast option, in spite of being a long detour, because it's high-speed for much of the way; but there's also a relatively direct but slow route via Nancy and Luxembourg, and possibily others). Don't worry about changing stations in Basel or Paris; in both cases the two stations are a few minutes' walk apart.
DB has fares starting at €39 for many international journeys if you book sufficiently in advance. I looked at a few fares with no age discount for travel on 27.10.2011 or the night before. You can combine a 79€ ticket from Basel to Brussels from DB (the cheaper fares are already out; the full fare is 145€) with a CHF 15.50 ticket from SBB from Zürich to Basel. There may well be a cheaper ticket if you break your purchase at a different point, choose a different route or travel on a different day.
If you're ok with a night train, you can take one from Zürich to Cologne. At the moment, DB quotes a fare of €49 from Zürich to Cologne on 26.10.2011 evening, arriving at the inconvenient but not completely indecent time of 5:42 in Cologne. Then the cheapest option DB offers is €24 to Brussels, leaving Cologne at 7:42 (time for a leasurly breakfast). I couldn't come up with a €73 ticket when I asked DB to go directly from Zürich to Brussels, which shows that introducing the right break in the ticket can help.