Definately fly one of the budget airlines to Phuket, and then take a boat to Phi Phi. Three budget airlines to check out are:
One Two Go
Just keep in mind that the flight is 1hr+, then another 45 minute taxi ride to the ferry on Phuket, then another 1 to 2 hour boat ride(depending on boat). So, even flying it can take half a day easy to get there. You can get really cheap flights, however. Watch out that your luggage is not overweight if you go with Air Asia, they will wack you for it. Good luck!