You are right in your comparison of income per capita between Israel, Turkey, and Mexico. However, the OECD is more interested in the contributions each country can do to the organization (economical cooperation). Therefore, the gross domestic product (GDP) is of more interest in this case to admit new members. As of 2008 (according to the International Monetary Fund), Mexico had the 13th biggest GDP in the world, Turkey the 17th, and Israel the 42nd. The OECD is NOT a ranking of countries by human development index (HDI), that must be clear to you. Yet, New Zealand has a lower GDP than Israel and is a OECD country, but perhaps they are in a better position to make higher contributions than Israel could.
Merkel says would welcome constructive Iranian role in any Syria talk (Reuters)
from the article: Israel set to become OECD's poorest member