Hey shivi, I have done a backpack trip. I can share complete itinerary to you with shopping and everything it costed around 45k per person involving air tickets, stay food, sight seeing entry fees. let me know if you would like to have the itinerary.
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Hey Shivi, You can check with Colorful Vacations Pvt Ltd.
They offer good quotation.
But I would suggest go plan yourself, read through few blogs and itenerary and plan. Thailand is a safe country and you can really enjoy your plan.