A taxi is 100 euros each way, a bus ride is...?
           Barny.
KTEL has started the summer schedule today. We have more busses from/to Rethymno and surprisingly many connections to Preveli now:
 Rethymnon - Plakias
 Monday - Friday
 06:15 09:00 11:00 14:15 18:00
 Saturday and Sunday
 09:00 11:00 14:15 18:00 
 Plakias - Rethymnon
 Monday - Friday
 07:00 10:00 12:00 15:00 19:00
 Saturday and Sunday
 10:00 12:00 15:00 19:00 
 Price Rethymno - Preveli = 4,50 €
Plakias - Preveli
 Daily
 10:00 11:00 12:00 18:00 19:00
 Preveli - Plakias
 Daily
 10:30 11:30 12:30 18:30 19:30
 Price Plakias - Preveli = 2,30 €
See the full schedule here:
http://bus-service-crete-ktel.com/timetable.php