Manila Taxi Rule.
Taxi drivers will undoubtedly try to cheat you. Expect it. One occasion as the price climbed to 1600 pesos per kilometer I just nodded my head. When we got stopped in a decent area in heavy traffic I got out and walked the opposite direction. There was nothing the guy could do.
I caught the next taxi to my destination. I've done that several times and now and travel to Manila with just a carry on for that reason.
I live in Davao. Drivers here are better but still try to cheat you. I know the roads but it always helps to snap a picture of the driver with your cell phone before getting in the taxi. I also snap the ID card as it is often different than the driver before I get in.