Cheapest airfare to the Philippines?

Im going to the Philippines in April and would like to get suggestions on where to get the least expesive ticket.I am also thinking of getting a one way ticket and buy a ticket there back to the US because Im not sure how long I need to stay there it could be a month up to 6 months maybe even longer. so as a green card holder would I get hassled for getting a oneway ticket at the customs? thank you!!!!