I looked up the Reservation cost for a trip from Brussels to Amsterdam, and it is apparently ~$50. Thats pretty expensive for someone who already has a pass. Are there any cheaper train options from Brussels to Amsterdam?
Yes, there's a normal IC train that does not require or offer reservations. On the station PA systems, you will hear it referred to as "the international train to Amsterdam/Brussels". Like Thalys, there's about one per hour. I don't know enough about rail passes (other than that they're usually a rip-off, especially for Belgium and the Netherlands) to know if there would be any extra fees involved for this train.
The bought-in-Europe Thalys 2nd class passholder fare is €39.00. The cheapest advance purchase online point-to-point Smoove fare ticket is €29.00. If you take an IC train, with your pass you can jump on any train and you'll pay nothing extra. The trip takes about one hour longer than with Thalys trains.
The Thalys train has the most expensive supplemental fees of any of the EU train systems. For a 1st class pass holder, it can be up to 100E. Says so right on Eurail website only the buyer of the pass is normally unaware of these special pages. Just take the IC train.