Jamieson, Fausset and Brown on Exodus 38:25

25. the silver of them that were numbered--603,550 men at half a shekel each would contribute 301,775 shekels; which at 2s. 4d. each, amounts to £35,207 sterling. It may seem difficult to imagine how the Israelites should be possessed of so much wealth in the desert; but it should be remembered that they were enriched first by the spoils of the Egyptians, and afterwards by those of the Amalekites. Besides, it is highly probable that during their sojourn they traded with the neighboring nations who bordered on the wilderness [Hewlett].