Travelling on ordinary (non-Thalys) trains the fare is EUR 39.20 in second class and EUR 63.60 in first. You can buy your ticket on the day, and travel on any train - just hop on and find a seat. The journey used to take only half an hour longer than using the Thalys service (and that will hopefully be the case again at the end of the year when they re-introduce direct trains), at the moment it takes about 45 minutes longer and involves a change.