For a simple quick way to get this achievement, start a new Rouge type character, and pour all your level up points through the opening of the game into Cunning and Coercion. I do not know what level of either you need to be at to do this, as I was level 4 Coercion, and 22 cunning when I found this.
After getting past Ostagar, and making your way through Lothering, head to Denerim as your first stop. Once hear you need to head to The Pearl, the brothel that is part of the quest Pearls Before Swine from the Sergent Kylon in the Market.
Once there, talk to the Sanga, owner of the Pearl, and ask to see either the men or women, it does not matter. When they come out, she goes through the rules, and tells you the price. You get a persuade option to drop the price to 20 silver instead of 40.
Now if you persuade her, select the option to change your mind. DO NOT ACCEPT AND PAY THE MONEY. If you accept the 30 silver price she offers, then the option to persuade her goes away. Turning it down and starting the dialogue up again adds another successful persuasion.