What's the cost of door alignment?

What is your question?

I purchased a 2010 STS in Oct,2015. Nice car. But I noticed that when one closed the rear door, either left or right, you get that solid "thump" sound. But when you close either front doors, there's a different sound in comparison to the rear doors. The sound of the front doors sounds like a type of rattling noise. Before I bought the car, I checked Car Fax and found out it had been in two accidents.When confronted, the dealership told me that everything was checked thoroughly with the car. I even took the car back during the initial warranty period to see about those doors. Do I need to have the car doors re-aligned and how much would that costs?