Accident v Non-accident

I purchase a certified used car from a dealer and was told that the car had not been in an accident. I found out some time later that infact it had.

As a percentage of the purchase price of a used car what is the difference between a car that has been in an accident and the same car if it had not been in an accident.

That all depends on the accident.  If it was not totaled and does not have a salvage title, there should be little if any difference.   I was once stopped at a traffic light when a cop hit me from behind.  Total repair cost to me $7.50 including labor to replace three shock absorbers (Styrofoam blocks).  Another accident might make the car good only for a few salvage parts.

Contact your insurance company and ask them. They understand the value of cars after an accident occurs. You will have to tell them exactly what was damaged and how badly to get an assessment. You would probably need the repair invoice to know for sure.