We visited those ports in March '02. Prices were best in Mazatlan, highest in Cabo, same stuff in all 3 ports. Do not pay what they ask, haggle downward, they still make money.
There is a shop in Mazatlan at the end of the street in the golden zone (can't remember the name) great for knock off designer bags. I got a wonderful purse and wallet, Louis Vitton knockoffs that looked really great, and have worn well. My husband bought a Mexican Rolex for about $60, but the minute hand fell off a few months later.
The street vendors selling "silver" on their arms on blankets, don't buy it, it's not real silver. Other shops and stall vendors have real silver at good prices (look for the official stamp, they'll show it to you).
Another good buy is Vanilla, very cheap at the grocery store and sometimes in other shops. Prices vary so look around