There are two kinds of trains on the Brussels to Amsterdam route. The fast Thalys trains use "airline pricing," meaning that advance purchase non-refundable tickets are cheap, and last minute tickets cost a LOT more. The price on the slower local trains is fixed, but they take longer and aren't as "nice." Either can work well, depending on your needs.
To see your options with prices, use the Belgian Rail international train site: https://www.b-europe.com/Travel. Look at prices for tomorrow and for a few months out, to see the discounts available for advance purchase.
Looking myself, I see that last minute tickets on the fast Thalys trains are about €80 each way, while advance purchase tickets are €29 for "non-flex" and €45 for "semi-flex." The slower trains are €42 each way, but these take about 3 hours and 20 minutes. Thalys tickets go on sale 3 months prior to travel dates.