The history of Ancient Greek coinage can be divided into three periods, the Archaic, the Classical and the Hellenistic.
The Archaic period extends from the introduction of coinage to the Greek world in about 600 BCE until the Persian Wars in about 480 BCE. The Classical period then began, and lasted until the conquests of Alexander the Great in about 330 BC, which began the Hellenistic period, extending until the Roman absorption of the Greek world in the 1st century BCE.
Like Ancient Roman coins Ancient Greek coins for sale on eBay usually sell for under $100. The less expensive Greek coins are less likely to be forgeries but be aware forgeries exist over the entire price range.
An ancient Greek coin’s value is ultimately determined by demand, availability, condition, and grading with the Gold and Silver coins unsurprisingly the most expensive.