I thought I'd answer the original poster's question. <g> The first known coin isn't any of the coins mentioned so far in this thread, not a Croesus stater, not a Miletos twelfth stater, not an Athenian Owl, not an Aegina Turtle. The Croesus stater is the closest, but coinage preceded Croesus in Lydia, which at the time was an independent, non-Greek kingdom in Asia Minor (present-day Turkey). Most numismatists regard Croesus' father, Alyattes, as having originated coinage, with the first coinage not being gold, or silver, but an alloy of gold and silver called electrum, and minted around 600 BC. Here's such a coin, a Lydian Lion, weighing about 4.7 grams and being about 54 percent gold:


There are differing views, with some numismatists believing coinage originated in China or India (particularly those from China and India), but Lydia has the strongest support in terms of archeology and ancient literature, though this support isn't absolutely conclusive. If you do a Google search for world's first coin you can find considerably more on this.
It is true that the Miletos silver twelfth stater (sometimes called a hemihekte or diobol) is the earliest "affordable" coin, the least expensive coin minted before the fifth century BC. It was minted by Greeks living near Lydia about a hundred years after the inception of coinage (500 BC, not 600 BC), with the most commonly mentioned dating being c. 525-494 BC, the ending date corresponding with the squashing of the Ionian Revolt of the Greek cities in Asia Minor (led by Miletos) by Persia.
The Athenian Owls and Aegian Turtles were more important coins, though more expensive today, the first widely used international coins, and the Athenian Owl is of further interest because of its association with Athens, the birthplace of democracy (at about the same time the first Owls were minted), because the basic Athena/owl type lasted for nearly a half millennium, and because the Owl was copied throughout the ancient world, with these Owl "imitatives" being the first coins in many locales.
If you're fairly new with this, the Miletos twelfth is a good choice, a 2,500-year-old coin less expensive today than many 150-year-old Liberty Seated dimes in about the same condition. Good hunting. <g>