Complex sentence: Despite high levels of wealth, a major factor affecting purchasing power is the high cost of living in Israel, which is higher than the OECD average, which is seen as having a significant impact on the middle and working classes.
Split into simpler sentences: Despite high levels of wealth, a major factor affecting purchasing power is the high cost of living in Israel, which is higher than the OECD average. This is seen as having a significant impact on the middle and working classes.