I'll only say,,, it cost $$ to keep people working and trucks on the road full of fuel, tires, commercial vehicle insurance,,, on the job liability insurance, workman's comp, (If you hire joe blow up the street and he gets hurt on your property and he has no insurance,, he can sue you) tools and assorted fittings and materials on hand /in stock to perform jobs that come in. I don't go and buy the stuff I need to do a plumbing job the same day of the job. If you want to save $ do the work yourself. $ 250.00-$300.00 is the average price to install a water heater in my area. This price is for a direct swap out assuming all vent pipe , wiring etc is up to code. If not the price goes up. I once got a call a about 10:00 pm once . Said he had been fooling around with his breakers and now his A/C wouldn't come on. I told him that there would a $95.00 service call + $90.00 per hr + any parts if needed. He said to come on it was hot and he couldn't stand it. I sent out 1 of my guys out to find that all he had to do was turn the A/C breaker back on... This guy pitched a fit because he was charged a $95.00 service call only !! in most cases , if the T&P valve is leaking on a water heater, You have a bad water pressure regulator or the water in the tank is getting too hot. Most T&P's are rater at 210* & 150psi. They will start to drip water way B-4 these temps & pressures are reached.