Expenditure is something that truly depends on you. How you travel. For a backpacker having 50k to 60k expenditure is enough.
Focus on traveling as much locations as you can. Also, Try and book hostel. Nowadays there good quality hostels available.
Flights:depends on multiple factors like
Try and find flights with one hop. There are cheap flights from Thailand. Thailand have many low cost carriers.
You and regularly search Websites such as
Kayak, skyscanner, wego and more...They will compare rates and will give good deals.
Lastly, If you want to travel cheap please avoid peak season.
Hope this helps you... Happy travel :)