With one connection in Paris, the cheapest I found on french voyages-sncf.com was a ticket for two passengers at about 170 EUR (210 CHF).
But I agree that usual fares are more in the 400 CHF range.
It would be worth to spend some time with a SBB-CFF employee or in a travel agency to find the cheapest train at the dates you plan to travel.
Alternatively, you may want to check bus with Eurolines .
One way tickets for two passengers are at about 180 CHF.
travel.stackexchange.com/users/241/mouviciel
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.
travel.stackexchange.com/users/277/Gilles 'SO- stop being evil'
You might also try to go by car pooling, e.g mitfahrgelenheit.de or mitfahrzentrale.de .
It costs around 6 cents per kilometre usually, or 70 CHF for a one-way trip.
You have to get lucky, though, to find a ticket.
Also, it might be worthwile to ask the DB (German rail company) for cheap tickets from Zürich into Germany and from Germany to Brussels or for a ticket from e.g. Freiburg (Breisgau).
They often have good offers, for example going from Freiburg (quite close to the German border) to Brussels for 79 Euro one-way.
travel.stackexchange.com/users/835/thiton
If you are taking any other trains in Central/Northern Europe during your trip, a Eurail pass may be worthwhile.
You could opt for a 3-country pass (Switzerland, Benelux and France or Germany, depending on your route).
Or, you could buy a Switzerland/Germany regional pass and pay for a single ticket to take you to Brussels from the border.
However, check beforehand if you need to pay extra for supplements on the trains on which you will travel (usually, this is not the case in Switzerland and Germany, but common for the high-speed trains such as TGV and Thalys).
travel.stackexchange.com/users/331/The Travelling Chicken
Another option to consider: Train to Basel (about 30 CHF per person if memory serves), then bus to the airport and a flight with Swiss from Basel.
You can book a trip all the way from/to the airport (including the bus) from the SBB.
The train ride is about 1 hour long, with the bus and connection, perhaps 1.5 or 2 hours in total.
Swiss is not per se a low-cost carrier (it's a remnant of the national carrier from Switzerland) but they often have cheap flights from Basel (ironically, to Brussels, it's a code share with Brussels Airlines, another barely surviving national carrier).
As of today, a quick search reveals prices starting at 43 CHF for one person for mid-june, not much more expensive than what you can expect from Easyjet or Air Berlin on similar routes.
Flying directly from Zürich on the other hand would cost at least four times as much.
Obviously, you have to be flexible on the dates to get the best price but even when adding the cost of the train, it can still be competitive as an alternative to Kloten and cheaper than 100 CHF per person.
Easyjet is also flying from Basel (but not to Brussels), so this also an option for other destinations, if you don't mind the hassle of an additional train/bus connection and can't afford the prices of flights from Zürich.
travel.stackexchange.com/users/6669/Relaxed
In the last week of October I plan to go to Brussels. I'm leaving from Zurich in Switzerland. I already did some research and tried to get a train ticket. The train is really fast (6.30 h) but a one-way ticket for 2 persons is more than 400 CHF :o So I also tried the budget airlines in Europe like easyJet or airberlin but nothing really cheap there.  Are there any other possibilities to get there?
Cheapest way to get from Zürich (Switzerland) to Brussels (Belgium)
travel.stackexchange.com
<budget><transportation><brussels><zurich><western-europe>
travel.stackexchange.com/questions/2497
travel.stackexchange.com/users/693/RoflcoptrException
The cheapest option on voyages-sncf.com  was a ticket for two passengers at about 170 EUR (210 CHF), with one stop in Paris. You could also talk with a SBB-CFF employee or travel agency, or search for cheaper connections yourself with SBB, SNCF, DB and SNCB. You could also buy a Eurail pass, fly from Basel, take a Eurolines bus or find a carsharing option at a website like mitfahrgelenheit.de or mitfahrzentrale.de. There are numerous options for travelling from Zurich to Brussels that are cheaper than a direct flight. Trains are cheaper if you are willing to change a few times. Mooreover, a Eurolines bus service is even cheaper than the train. Finally, flights from Basel to Brussels are generally cheaper than from Zurich. There is a train service from Zurich to Basel.
There are many railway companies operating and a cheap ticket from Zurich to Brussels may be found if you shop around and are willing to change a few times. Eurolines bus service is a cheaper alternative to the train. Carpooling is an option. Flying from Basel is cheaper than Zurich in genereal and there is a train from Zurich to Basel.