The figure has varied fairly wildly over the last decade, so I think it would be hard to pin down an accurate number. The average seems to be about $100-200/month. I don't think that makes his actions any less humble though.
You should take note though that the focus here is on factory workers, whereas the average Chinese wage for all Chinese workers would be lower.
Actually, Hangzhou is a very rich city by Chinese standards due to their entrepreneurial culture. Despite "only" having 2 million inhabitants, LV sells the most items in Hangzhou compared to all other Chinese cities. Also, their more central streets are lined with luxury car shops.
$20/day does not seem too unreasonable for local Hangzhounese.
You should take note though that the focus here is on factory workers, whereas the average Chinese wage for all Chinese workers would be lower.