Can anyone tell me the cost per day for running the electric to the coolant plug on the duramax diesel.

Do you mean the block heater?

I would guess somewhere in the $3-$4 range per day, but why would you ever worry about it in Florida?

Yikes! What kind of electric power rates do you have?
Here in overpriced Kalifornia that would be 25-30 KW worth of electric power.

Dont even need to think about the heater till you are close to 0*f

Yes, I meant to say the block heater. My son in law own's a place where there are a lot of diesel's park at night time. He finds that the electric bill in the winter months is getting higher. He wants to be able to charge the people who plugged in the block heater a higher bill because he pay's the electric. This is in Delaware , where the night's were in the low 20's and 10's. If the rate's are $3 to $4 like coyoteKid says, my son in law is losing a lot of money. I didn't know if someone has put a amp-meter on, to find this out or not.

We pay around 6¢ per kw here. I think the heater is 600 watts, but let's say it's 1000. So every hour would cost 6¢ per truck. So if it's parked 10 hrs, then it is 60¢ per night.