Myanmar inflexible bus prices are changing depends on the conduction. There are
different prices for local and foreigner. It changes often because of the inflation and the fuel price in Myanmar. Other reason is the special festival and holiday season in Myanmar. The inflexible bus prices if you stop on the way From Pa-an for example, it cost the same (K 600) to go to Kyaiktyo (3 hours) or to Yangon (12 hours). Bus Myanmar prices from Yangon to Mandalay and Yangon to Beaches (Chaungtha Beach, Ngwe Saung Beach) goes up during water festival (Myanmar New Year).
It is always good if you ask how many places they stop and how long before you buy your bus myanmar ticket. But what they said is only 50% correct.