I need to fly to Basel (CH) soon. I have checked fares to Basel/Mulhouse from Brussels. The fares are ridiculous expensive (>900 Euro for an 1 hour flight). I can take the night train, but before considering that I was wondering if there are other airports I could use when I need to be in Basel?
You can easily fly to Zurich. The connections from Brussels to Zurich are way better than the connections from Brussels to Basel. And from Zurich you're in less than a hour in Basel when you take the train. The prices for that train travel start around 15 Swiss franks. The SBB (Swiss national train carrier) has an understandable website. I found flights for around 300 euros for next week (with Swiss)
You could also fly over Geneva to Zurich which is cheaper (around 200 euro). Then I would think about only take the first leg of the flight, leave the plane in Geneva and take the train. But this really depends on the actual date and time you're going to fly.
You could also fly to Geneva, but this is a little bit further away from Basel. The train takes almost 3 hours.
There are also other airports nearby which may be useful (even if I doubt it). This are particularly Friedrichshafen and Altenrhein.
I would look at flying to Zurich and taking a train
Bern might be another option but may not be as good. Personally I'd drive but that may be US traveler talking.