There are many roadside shops. Any trip inland and you pass various shops selling crafts, jewelry, sandals ,etc.
We seen a shop that had some wood crafts. Among the things for sale was this headboard for a bed. This headboard was about 7 - 8 feet wide. Very nice but $$ to ship home.
What to buy: If you can afford it, very nice wood crafts..
On the way to Copala you will pass many craft shops and places to buy all sorts of trinkets and souveniers. You even get to see them making their stuff!!! There is wood carvings, furniture, pottery and many other items that are cheap and unique. There is a place where they make decorative tiles and a bakery inside a house...All very interesting and worth the trip.
What to buy: Anything uniqie and interesting...but if you are flying home it needs to fit in your suitcase!!
What to pay: It varies on what u buy
If you like to sew or do crafts be sure to shop the numerous fabric stores in Mazatlan. They are everywhere and the prices are much cheaper than in the states.