June, July, August is "off season", you'll get a better deal.
Heard that travelling by ship to Aklan or via Batangas is possible, much cheaper but longer time.
There is an overnight domestic ferry (MBRS) who ply this route once a week. departing from Manila North Harbor, Pier 8. You may want to check on how safe it is.
+63-2-243-5888
This is the latest, go to Batangas pier and take ferry to Calapan.
From Calapan there are minibus going to Caticlan.
From Caticlan you take a boat to Boracay.
Be warned, it's a long travel accdg. to sister.