I agree 100%. If you act like you don't know shit they'll price jack you. It's like being on vacation somewhere and telling some gypsy cab driver you're from out of town. He'll definitely take the detour/scenic route to your destination.
Also bartering is fair game and commonplace south of the border. If they offer you something for $45, tell them all you have is $40 and they'll usually bite. I also used to throw a $20 bill at whoever waited on me after I bought my stuff. Next time you go there you'll get treated like a white man. One time I went in and they were out of Sos 250. I was like, fuck! The owner comes out and tells me to wait 15 minutes and he'll have some. 10 mins later some 9 yr old kid rolls up on a rusty bike with a brown paper bag of sos 250's. It's trippy down there-like a whole different world.