Don't go to the place directly across from the fountain outside the 'middle' entrance of the Costanera. It's bloody horrible, I had a chori there last week that was the greasiest thing I've ever tasted.
I will have to check out the one recommended here though, as well as the one someone else mentioned on Carlos Calvo, which is right round the corner from me.
Oh, but if it's 'only' $6 I think I've been lucky - I've had loads, never from the same place, in the two months since moving here and I can't remember paying any price other than $6 for any of them. Of course, first time I was here in 2003 it was even cheaper than that...