There are some really nice and expensive hotels in Antwerp, but there is also a lot of crap, especially in the seedier parts of the town.
If you prefer a good and clean hotel I would recommend the Ibis Hotel. It is located in a decent part of Antwerp and not too expensive (approx BEF 3000 per night, I think). Nearer Central Station you can find other 3-4 star hotels but they'll be more expensive.
If this sounds expensive you could try the hostels. One place I have heard about is new and clean, is near central Station and has free internet. This is not really a hostel but rather a cross between a hostel and hotel. They cater for the better-off students staying short or longer term. I haven't been there myself but a friend did and she liked it. If this is your cup of tea check out their website,
http://www.vremco.com.