Currently Schiphol-Brussels is about 2 hours 20 minutes, with 2 trains an hour. Price is complex for international journeys, but I'd say that for an open 2nd class return you'd be looking at around 60 quid. If you can take some restrictions, you can probably bring that closer to 40.
When the much-delayed HSL Zuid line (TGV standard) opens later this year, the time will come down to around 1 hour 25 minutes, again with 2 trains an hour. But fares will rise. The talk is that Dutch domestic trains on the line will cost about 25% more. I've no idea how that applies to international trains.
Brussels is also about 90 minutes from Charles de Gaulle by direct train, but those are more expensive and a lot less frequent.