I've recently required the need to have more sand bags in my space, so I purchased 20 pcs of these 9" x 10.5" sized Sand Bags. Unless you see them in person, shopping for 'inexpensive sand bags' on the internet might be a hit or miss. All you get is a few photos and not much information, so I thought I would share what i'm using.
The product listing doesn't really provide the exact weight these will be when loaded up, but from my estimates it should be around 20 lbs. each. The material is good lined with a tightly woven plastic like material on the inside so the sand doesn't sift it's way out. Stitching is great and double zippered to prevent them from accidentally opening up - pretty standard quality bags. Now time to make a stop over at the hardware store and pick up a few hundred pounds of Play Sand (for kids sandboxes) and fill them up. Here's a listing to the exact same 9" x 10.5" Sand Bags i've ordered (here).
10pcs of 9" x 10.5" Sand Bags