There may be buses which would be cheaper for this, but train will be so much better for this I wouldn’t even look at anything else.

Probably train. There are at least a few vendors selling train services between Brussels and Amsterdam—Thalys, Eurostar, Belgian Rail—but currently the cheapest $56 from TGV Europe. I found this using Momondo.com one-way search for May 6. Momondo primarily is a cheap flights aggregator, but often show rail routes within Europe too.

update= I checked further and found it for only $34 from www.nshispeed.nl (under 26 price). $51 non-youth full price.

update 2 (based on OL’s suggestion) = even better… 10EUR (7EUR promo fare + 3 EUR reservation fee), http://www.eurolines… Takes 4 hours though, vs. about 2.5 hours with the train.

Train is the fastest! Busses from Antwerp to Amsterdam come pretty cheap as well, if you book it in advance, see Eurolines.com

BTW: I see you plan to go to Brussels and Antwerp, but skip Bruges. You really should visit Bruges, it is much more interesting than those two cities, and also the prime tourist destination of Belgium. Skip either Brussels or Antwerp.