Here is how you do the maths
Eurail Benelux pass, 2nd class, 2 persons, 4 days in a month, €322 (or, €40 per person per day)
source: http://www.eurail.com/eurail-passes/one-country-pass/benelux
If you have a pass, going to Amsterdam to Brussels by Thalys train, the supplement which you must pay is €30 (could be more), bringing your ticket cost to €70 per person
source: http://europetrainsguide.com/Advice/Rail-Passes/Rail-passes-%E2%80%93-to-buy-or-not-to-buy.html
If you are buying point-to-point, buying normal advance purchase ticket from Thalys, (Amsterdam to Brussels, or Amsterdam to Bruges with the option of All Belgian station), within 90 days of booking window, will cost you as low as €36 per person. The same applies for returning from Brussels to Amsterdam. Between Belgian cities, ticket will cost about €14, much lower than the price you paid using a pass