How much is a 5 lb gold nugget worth?

I'm curious to know, what would be the approximate value of a 5 pound gold nugget in today's market? Given the fluctuations in the price of gold, is there a reliable way to estimate its worth? And how does the purity of the gold nugget factor into its overall value? I'm eager to understand the intricacies involved in determining the monetary worth of such a precious commodity.