Don't ask me why. Fuel is way cheaper in Davao City. I was there just this weekend and diesel costs only P26-P27 a liter. In Gensan, the same would cost around P30/L and in Koronadal P31/L. I've read somewhere that diesel in Metro Manila costs between P28-P30/L.
I forgot to take note of the prices of the other fuel products but I am sure these are lower as well. So if you are from anywhere near (e.g. Soccsksargen) and are planning to go to Davao in your car, load up just enough to get you to the city. If your car has a 60-liter fuel tank, you can save a lot when you gas up full on your return trip.
If you are coming in from Manila... take the plane :)
POSTSCRIPT: Fellow Soccsksargen blogger Donna Mae says gas is even cheaper in Cagayan de Oro and Iligan!