There is a number of tables of the richest prizes captured by various pirates during various periods in J.Rogozinski's "Dictionary of Pirates" (Wordsworth Editions 1997). There are some figures:
Caribbean and South America's Pacific Coast, 1628-1686:
(the greatest) P.Hein commanding 31 vessels and 2300 men - 11-14 million Dutch guilders,
F.L'Ollonnais with 8 ships and 660 men - 260000 pesos=65000 pounds,
H.Morgan with 10 small ships and 500 men - 250000 pesos=62500 pounds,
etc.;
Indian Ocean and Red Sea, 1690-1722:
(the greatest) J.Taylor and O.La Bouche - 875000 pounds, being 3600 pounds per share,
H.Every and his squadron - 325000 pounds, being 1000 pounds plus jewels per man from Every's "Fancy" (who took most of the loot),
E.Condent - 150000 pounds, being 2000 pounds per share,
etc.
These are the maximum, the minimum being nothing at all, and a death in the first battle, so it's hard to calculate a mean sum
There are more figures in the book, so I'll put them here too if you wish.