It depends on you. your accommodation, mode of transportation will basically vary on cost. If you are a budget traveler and can manage staying in hostel and transport by local transporter then 20k is enough.
I am just traveling next week... All bookings are done... 12k this includes flights from Calcutta to Bangkok and return
Bangkok to phuket flight and return
Expecting it to be around 20k
Traveling is something which truly depends on how you want to travel... Happy travel:)