ko pan yang ( excuse the spelling ) is ace if u intend to go around the new year time for the full moon parties
I went last year to krabi with my m8's for a month, went mainly for the rock climbing but there were still loads of parties & fit women mainly aussies & kiwis plus loads of brits
ko phi phi is cool 2 tried some scuba diving while we were there
total cost for the month was about £1400 for the month inc flights & accomodation & all the spending money
food is stupidly cheap & fantastic

cheapest way to go is to just get the flights & stay in back packers hostels & chalets. negotiate the prices when u get there.