The Persians vs. The Greeks

Several factors could've affected the Persians belief that the Greeks weren't "worth their time". The most obvious one that relates most to common human nature is arrogance. The immense size of the Persian Empire was the largest the world had yet seen, however, with dominance often comes conceit and the belief that one is superior to another. This arrogance of the Persians was one of the factors that lead to their eventual demise.
The Greeks were also not a united empire and were therefore probably not seen as that advanced in the eyes of the Persians. Through taxation of the civilians, the Persian empire was able to construct an empire with exceptional infrastructure and by dividing the empire into 20 provinces, governing became easier and the empire was able to stay cohesive through seemingly loose ties with the bureaucratic headquarters at the center of the empire. However, this complex bureaucracy of the Persians was completely opposite the governing systems of the city-states of Ancient Greece. These city-states had been primarily founded upon the fundamental necessity of democracy. This difference may have been seen as a weakness to the Persians because they realized that their system of government was of relatively higher caliber.
The city states of Ancient Greece were also in constant war with one another. Sparta and Athens are the classic example of the perpetual conflict between the individual city states. From an outsiders perspective this is one of the qualifications of inferiority between what could be worse that instability among ones own people?
Also, in a very general sense, Greece was a relatively resource-poor region and their economy depended on raw materials from other cultures. It is also possible that the Persians found the Greeks to be a waste of time because the amount and quality of the goods traded wouldn't be as preferable.