Which website are you using? You can actually book Brussels - Amsterdam on (at least) three different websites:
SNCB (Belgian Railways):
www.b-europe.com/Travel
NS International (Dutch Railways):
www.nsinternational.nl (known as nshispeed.nl until very recently)
Thalys (who actually operate the high speed trains on this route):
www.thalys.com/be/en
All three seem to be currently offering the 11:52 departure on 28th September (direct Thalys Brussels to Amsterdam) for €29. However, I doubt that price will remain available for long...
€59 - €79 seems more usual for the Thalys on this route (especially when booking at fairly short notice). So, I tend to use the classic service. As
Aviatrix and
Gagravarr have said, this is only €39.20*, even if purchased on day of departure, and is valid on any classic train (ie: non-Thalys, but including IC Direct within Holland) on the given day. Admittedly the journey is ~1hr longer, but there are (almost) hourly departures, compared to two-hourly for the Thalys. Consequently, as the classic tickets are flexible (unlike the 'cheap' Thalys tickets), unless you can be sure of timing your arrival at the departure station to coincide with the Thalys schedule (or are willing to pay for a flexible Thalys ticket), it can often be quicker/easier just to take the classic service.
Some schedules for the classic service (taking 2h40 for Brussels-Amsterdam) involve a 2min connection in Rotterdam. This may seem optimistic - but it's normally cross-platform (11 to 12), and IME they will hold the IC Direct train if the train from Brussels is a few minutes late.
* Or €29.50 if you have a RailPlus card