Most feudal systems weren't based on wealth, they were based on military accomplishment. Valorous or talented commoners who caught the attention of their lords could be knighted, and exceptional knights could be given land, making them part of the aristocracy.

Which was pretty much the exception. Nobles were practically always born from the aristocracy.

Then again, this is largely true in many free, democratic and capitalistic countries even today. Social mobility in countries like the United Kingdom and the United States of America is nowhere near as great as people would like to think.