How much weight can a Ziploc bag hold?

One gram would be about right for a sandwich size bag, however, zip lock bags come in all sizes and thicknesses.

Standard zip lock bags use plastic that is about 2 mil thick.

There is a huge market for zip lock bags that are 4 mil thick or heavy duty (as in a freezer bag.) So those bags weigh twice as much.

There are huge zip lock bags designed to hold laundry. (used to seal up bedbug infested clothes. Stick these bags in the freezer with the bedbug or lice infested clothes and they will die. The seal keeps them from escaping.)

Zip lock bags come in all sizes and thicknesses. If you want to see a big selection from a reputable seller go to;

superpolybags.com

Check the shipping weight of the item. Divide by the amount of bags in the case. You'll get a good approximate weight fot each bag.