It looks to me like the sand bag is close in volume to the statue. As you've pointed out, the density of sand is quite different from the density of gold. Different kinds of sand have different densities, but 2.3 g/mL is a typical value. Gold has a density of 19.3 g/mL, so he needed a volume of sand that was (19.3 g/mL)/(2.3 g/mL) = 8.4 times the volume of the statue. Jones actually took sand away, when he should have added it.
The statue fit in one hand easily. I'd guess it was somewhere around 1 L in volume. That would put its weight at around 19 kg, or about 42 lbs. They were tossing it about as though it weighed much less than that when they were running through the tomb, so perhaps it was hollow, or just coated in gold.
(In our next episode, we consider the capacitance required for the Ark of the Covenant to produce a bolt of lightning capable of melting two dozen Nazis...)
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