What is the weight of a 250ml empty glass beaker?

The weight of the beaker will vary depending on what it is made of, i.e. the type of glass, the thickness of the walls, etc. The only way to determine this is to actually weigh the beaker. It cannot be calculated.
250 grams because water density is the same for ML to GRAMS.
No if it was filled with water to 250 ml the water would weigh 250 grams. The beaker would weigh whatever, depending on the thickness of the glass.