Upvote:2
Based on your other question, I guess you want to travel from Köln Airport to Maastricht station.
Taking the train from Köln Airport to Aachen will cost you €16,80 (bahn.de), and then from Aachen you can take the bus 50 to Maastricht for €5,50 (Veolia Limburg). Bus 50 leaves just outside the station, timetables can be found on 9292.
A cheaper option might be a ride-share ("Mitfahrgelegenheit"), depending on availability of someone driving this exact route on the day you want. Two sites offering ride-share are here and here. When I checked, offers were around €8, however, this will not include the ticket from Köln Airport to where the agreed meeting point will be.