Cost for a boat trip on Lake Luzerne

http://www.lakelucerne.ch/en/timetable-fares/timetable/

It depends which trip you take. There isn't just one, the boats are public transport on the lake and serve many locations the whole length of the lake.
It also depends if you have a Swiss Pass, half price card or other discount card, all of which are valid.
You can also go one way by boat, and come back by train.
The link Jazz+Travels posted was for the timetables: http://www.lakelucerne.ch/en/timetable-fares/timetable/

The fares are here: http://www.lakelucerne.ch/fileadmin/lakelucerne/content/Dokumente_Downloads_PDF/01_Fahrplan-Preise/02_Preise/2018/2018_Fares_and_conditions.pdf

This map of the lake will show you where all the places in the fare list are: http://www.lakelucerne.ch/fileadmin/lakelucerne/content/Dokumente_Downloads_PDF/02_Angebote/04_Imageprospekt/2016/Discover_Lake_Lucerne_by_boat.pdf