One thing I have discovered is that you can actually save 10-30% of ticket cost if you change the site location of your booking. This applies to Jetstar site so far in my experience..
What to do:
1. Go to the jetstar website https://www.jetstar.com
When you go the jetstar website you will redirected based on your location.
Example if live in Vietnam so you will be directed to the vietnam site: https://www.jetstar.com/vn/en/home
2. Enter the necessary details on the page. Example for this query is a one way flight from Singapore to Da Nang on July 11, 2018.
After selecting the travel date and number of passenger press "Select Flight Time"
3. A list of flights will be presented to you. In your browser text line update the 2 letter country designation (marked in red). HOPEFULY you will notice a difference of price for your booking. ;-)
Result: 136SGD (27.6% Saving)
4. Use the site where the best price exist and proceed with the booking.
Good luck on your ticket savings hunting. :)