Cheap flights are only available from Kuala Lumpur, not only to Bali. Check with skyscanner flights from BKK and KUL. Happy I live in south of amazing Thailand. Can easy take a comfortable "high speed train" from Padang Besar to KUL - 5 hr trip
If you fly from Bangkok quite expensive. As I live in south I fly from Kuala Lumpur. I fly regularly and average return ticket is MYR 240 (~2400 THB). Malindo air have full service (30kg luggage, meals, entertainment screen in every seat). Depart from KLIA main terminal and not from Air Asia cattle terminal