[wylos]

cheapest way is not always the best way......depends if ya like long coach or train journeys,and if u got a bit of time on your hands,as these take up a good day if not longer .
i.e phuket to chang mai by 'vip'bus .........phuket-bangkok 12 or so hours....bangkok to chang mai 12 hours...cost around 800-1000b.

train is another option,i prefer,more space, can walk about,sleeper as well if u book ,2nd class fan was fine for me,bottom bunk a bit bigger if i remember correctly,was comfy,if u take this option,it will be train cm- bkk 12 hours ,bkk-surathani 10 hours ,then u have to bus it to phuket which takes around 3-4 hours,bear in mind there will be waiting around aswell on top.cost around 1000b.

or u could save yourself time and hassle and just get the plane.
as u dont know exact dates u wont be able to book in advance and poss get some cheaper deals......so......

cm-bkk 2500b
bkk-hkt 2000b

so could look at it this way.....4500b+ for flights
1000 b for train or plane.

time and money is what u gotta weigh up.
3500 b can get u a long way in LOS.
i suppose its personal preference at the end of the day as well

hope this helps.