Western Sahara receives something like $800 million per year in subsidies from Morocco, according to the 2011 United States diplomatic cable leaks. That's a lot of money for a region with a population of only around 600,000 people and with a GDP per capita of only around $2,500. In fact, that means that the subsidy is well over half the total economy, which makes it one of the largest subsidy programs in history (
source).
Even if petroleum deposits that are believed to be in the region were discovered and developed it is thought that the area would likely still be economically nonviable. The region doesn't even have much water outside of the occasional oasis, and the primary economic activity is goat herding by nomadic peoples.