How much should i pay for a 2009 dodge charger sxt with 20k miles on it?

"Book" prices mean nothing. Dealers don't use them and neither does anyone else who knows the market. You need to do a couple things. First, read the owner reviews on the car to insure it is what you want. Second, look at many different sources for the actual selling prices. AutoTrader, Craigslist, Yahoo Autos and eBay would be a good start.